1. Guidelines for vets, farm managers, and
event organisers
Christine King BVSc, MANZCVS (equine), MVetClinStud
Anima Vet, Park Ridge, QLD animavet.com.au
Copyright © Christine M. King, 2021. All rights reserved.

2. “When the risk and consequences of a systemic vaccine-associated adverse event
exceeds the risk and consequences from the disease, it is rational to attempt to
predict immunity by serologic testing to either eliminate vaccination for the disease
in question or extend the revaccination interval for as long as possible. ”
AAEP Guidelines for serology in horses with adverse events from vaccination. American Association of Equine Practitioners (AAEP), 2020.

3. § Many owners don’t want their horses vaccinated against Hendra virus (HeV)

4. § Many owners don’t want their horses vaccinated against Hendra virus (HeV)
o concern about vaccine safety is prevalent and outweighs concern about the virus itself

5. § Many owners don’t want their horses vaccinated against Hendra virus (HeV)
o concern about vaccine safety is prevalent and outweighs concern about the virus itself
o other reasons include low incidence of HeV (perception of low risk) and cost

6. § Many owners don’t want their horses vaccinated against Hendra virus (HeV)
o concern about vaccine safety is prevalent and outweighs concern about the virus itself
o other reasons include low incidence of HeV (perception of low risk) and cost
§ Many owners of vaccinated horses don’t want to keep up with annual HeV boosters

7. § Many owners don’t want their horses vaccinated against Hendra virus (HeV)
o concern about vaccine safety is prevalent and outweighs concern about the virus itself
o other reasons include low incidence of HeV (perception of low risk) and cost
§ Many owners of vaccinated horses don’t want to keep up with annual HeV boosters
o concerns include vaccine safety, cost, and whether annual boosters really are necessary for protection

8. § Many owners don’t want their horses vaccinated against Hendra virus (HeV)
o concern about vaccine safety is prevalent and outweighs concern about the virus itself
o other reasons include low incidence of HeV (perception of low risk) and cost
§ Many owners of vaccinated horses don’t want to keep up with annual HeV boosters
o concerns include vaccine safety, cost, and whether annual boosters really are necessary for protection
o horses who have had vaccine reactions or other adverse effects of the HeV vaccine are an important
subset; these horses should be boostered with great care, at an extended interval, or not at all

9. § Many owners don’t want their horses vaccinated against Hendra virus (HeV)
o concern about vaccine safety is prevalent and outweighs concern about the virus itself
o other reasons include low incidence of HeV (perception of low risk) and cost
§ Many owners of vaccinated horses don’t want to keep up with annual HeV boosters
o concerns include vaccine safety, cost, and whether annual boosters really are necessary for protection
o horses who have had vaccine reactions or other adverse effects of the HeV vaccine are an important
subset; these horses should be boostered with great care, at an extended interval, or not at all
§ Hendra antibody tests can be used as proof of immunity rather than relying
solely on proof of vaccination

10. § Antibodies are specific proteins the immune system produces in response
to a pathogen (disease-causing agent) or vaccine

11. § Antibodies are specific proteins the immune system produces in response
to a pathogen (disease-causing agent) or vaccine
o Hendra virus (HeV) is an example of a pathogen

12. § Antibodies are specific proteins the immune system produces in response
to a pathogen (disease-causing agent) or vaccine
o Hendra virus (HeV) is an example of a pathogen
o the HeV vaccine contains a protein (‘G glycoprotein’) HeV has on its outer surface that the virus
uses to attach to the horse’s cells in order to gain entry

13. § Antibodies are specific proteins the immune system produces in response
to a pathogen (disease-causing agent) or vaccine
o Hendra virus (HeV) is an example of a pathogen
o the HeV vaccine contains a protein (‘G glycoprotein’) HeV has on its outer surface that the virus
uses to attach to the horse’s cells in order to gain entry
§ Antibodies are designed to protect against disease when the body next
encounters that pathogen

14. § Antibodies are specific proteins the immune system produces in response
to a pathogen (disease-causing agent) or vaccine
o Hendra virus (HeV) is an example of a pathogen
o the HeV vaccine contains a protein (‘G glycoprotein’) HeV has on its outer surface that the virus
uses to attach to the horse’s cells in order to gain entry
§ Antibodies are designed to protect against disease when the body next
encounters that pathogen
o the HeV vaccine stimulates production of ‘neutralising’ antibodies against HeV’s G glycoprotein

15. § Antibodies are specific proteins the immune system produces in response
to a pathogen (disease-causing agent) or vaccine
o Hendra virus (HeV) is an example of a pathogen
o the HeV vaccine contains a protein (‘G glycoprotein’) HeV has on its outer surface that the virus
uses to attach to the horse’s cells in order to gain entry
§ Antibodies are designed to protect against disease when the body next
encounters that pathogen
o the HeV vaccine stimulates production of ‘neutralising’ antibodies against HeV’s G glycoprotein
o these antibodies protect the horse against HeV infection and infectivity — illness and transmission

16. § However, the HeV vaccine causes adverse effects in some horses

17. § However, the HeV vaccine causes adverse effects in some horses
o they may react to the G glycoprotein (the core vaccine component) itself or to one or more
of the additives...

18. § However, the HeV vaccine causes adverse effects in some horses
o they may react to the G glycoprotein (the core vaccine component) itself or to one or more
of the additives...
o the proprietary adjuvant (a substance added to ‘goose’ the immune response to the core component),
which can result in an exaggerated inflammatory or allergic response

19. § However, the HeV vaccine causes adverse effects in some horses
o they may react to the G glycoprotein (the core vaccine component) itself or to one or more
of the additives...
o the proprietary adjuvant (a substance added to ‘goose’ the immune response to the core component),
which can result in an exaggerated inflammatory or allergic response
o thiomersal (aka thimerosal), a mercury-containing preservative and potential allergen

20. § However, the HeV vaccine causes adverse effects in some horses
o the manufacturer doesn’t say which adjuvant(s) is used, but a research paper documenting
development of a HeV soluble G protein subunit vaccine discussed these two adjuvants [1]:
[1] Broder CC, Weir DL, Reid PA. Hendra virus and Nipah virus animal vaccines. Vaccine 2016; 34: 3525–3534.

21. § However, the HeV vaccine causes adverse effects in some horses
o the manufacturer doesn’t say which adjuvant(s) is used, but a research paper documenting
development of a HeV soluble G protein subunit vaccine discussed these two adjuvants [1]:
o Alhydrogel™(aluminium hydroxide gel), which boosts the antibody-mediated immune response
to the core vaccine component
[1] Broder CC, Weir DL, Reid PA. Hendra virus and Nipah virus animal vaccines. Vaccine 2016; 34: 3525–3534.

22. § However, the HeV vaccine causes adverse effects in some horses
o the manufacturer doesn’t say which adjuvant(s) is used, but a research paper documenting
development of a HeV soluble G protein subunit vaccine discussed these two adjuvants [1]:
o Alhydrogel™(aluminium hydroxide gel), which boosts the antibody-mediated immune response
to the core vaccine component
o CpG, which is a short, single-stranded, synthetic DNA molecule that is used in vaccines as an
immunostimulant
[1] Broder CC, Weir DL, Reid PA. Hendra virus and Nipah virus animal vaccines. Vaccine 2016; 34: 3525–3534.

23. § However, the HeV vaccine causes adverse effects in some horses
o the manufacturer doesn’t say which adjuvant(s) is used, but a research paper documenting
development of a HeV soluble G protein subunit vaccine discussed these two adjuvants [1]:
o Alhydrogel™(aluminium hydroxide gel), which boosts the antibody-mediated immune response
to the core vaccine component
o CpG, which is a short, single-stranded, synthetic DNA molecule that is used in vaccines as an
immunostimulant
o CpG mimics the presence of pathogens such as viruses; it has a 'pathogen-associated molecular
pattern’ (PAMP), so the body reacts to it as it would to invaders such as viruses
[1] Broder CC, Weir DL, Reid PA. Hendra virus and Nipah virus animal vaccines. Vaccine 2016; 34: 3525–3534.

24. § However, the HeV vaccine causes adverse effects in some horses
o the manufacturer doesn’t say which adjuvant(s) is used, but a research paper documenting
development of a HeV soluble G protein subunit vaccine discussed these two adjuvants [1]:
o Alhydrogel™(aluminium hydroxide gel), which boosts the antibody-mediated immune response
to the core vaccine component
o CpG, which is a short, single-stranded, synthetic DNA molecule that is used in vaccines as an
immunostimulant
o CpG mimics the presence of pathogens such as viruses; it has a 'pathogen-associated molecular
pattern’ (PAMP), so the body reacts to it as it would to invaders such as viruses
o over-reaction to such vaccines can occur in horses whose immune systems are already primed
to recognise and respond vigorously to PAMPs, whether natural (virus) or synthetic (vaccine)
[1] Broder CC, Weir DL, Reid PA. Hendra virus and Nipah virus animal vaccines. Vaccine 2016; 34: 3525–3534.

25. § However, the HeV vaccine causes adverse effects in some horses
o vaccine site reactions and other adverse effects are more common than reported

26. § However, the HeV vaccine causes adverse effects in some horses
o vaccine site reactions and other adverse effects are more common than reported
o with any vaccine, adverse effects are not always recognised as such, and most vaccine reactions
are not reported

27. § However, the HeV vaccine causes adverse effects in some horses
o vaccine site reactions and other adverse effects are more common than reported
o with any vaccine, adverse effects are not always recognised as such, and most vaccine reactions
are not reported
§ a Class Action lawsuit against Zoetis (HeV vaccine manufacturer) is currently
in federal court

28. § However, the HeV vaccine causes adverse effects in some horses
o vaccine site reactions and other adverse effects are more common than reported
o with any vaccine, adverse effects are not always recognised as such, and most vaccine reactions
are not reported
§ a Class Action lawsuit against Zoetis (HeV vaccine manufacturer) is currently
in federal court
o it essentially claims that Zoetis overstated the risks of HeV and understated the risks of the HeV vaccine

29. § However, the HeV vaccine causes adverse effects in some horses
o vaccine site reactions and other adverse effects are more common than reported
o with any vaccine, adverse effects are not always recognised as such, and most vaccine reactions
are not reported
§ a Class Action lawsuit against Zoetis (HeV vaccine manufacturer) is currently
in federal court
o it essentially claims that Zoetis overstated the risks of HeV and understated the risks of the HeV vaccine
o potential for harm (including death) is a pervasive reason why horse owners are reluctant or refuse
to have their horses receive the HeV vaccine or continue with annual boosters

30. § However, the HeV vaccine causes adverse effects in some horses
o vaccine site reactions and other adverse effects are more common than reported
o with any vaccine, adverse effects are not always recognised as such, and most vaccine reactions
are not reported
§ a Class Action lawsuit against Zoetis (HeV vaccine manufacturer) is currently
in federal court
o it essentially claims that Zoetis overstated the risks of HeV and understated the risks of the HeV vaccine
o potential for harm (including death) is a pervasive reason why horse owners are reluctant or refuse
to have their horses receive the HeV vaccine or continue with annual boosters
§ There is a way for us to have our cake and eat it, too ...

31. § A field study showed that annual boosters are unnecessary in most horses [1]
[1] Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses following vaccination with Equivac® HeV: a field
study. Australian Veterinary Journal 2018; 96: 161–166.

32. § A field study showed that annual boosters are unnecessary in most horses [1]
o the protective threshold for HeV antibodies is yet to be determined
[1] Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses following vaccination with Equivac® HeV: a field
study. Australian Veterinary Journal 2018; 96: 161–166.

33. § A field study showed that annual boosters are unnecessary in most horses [1]
o the protective threshold for HeV antibodies is yet to be determined
o in an experimental vaccine-challenge study, horses with titres of at least 16 (the lowest level tested)
were protected [2]
[1] Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses following vaccination with Equivac® HeV: a field
study. Australian Veterinary Journal 2018; 96: 161–166.
[2] Middleton D, Pallister J, Klein R, et al. Hendra virus vaccine, a One Health approach to protectin horse, human, and environmental health.
Emerging Infectious Diseases 2014; 20: 372—379.

34. § A field study showed that annual boosters are unnecessary in most horses [1]
o the protective threshold for HeV antibodies is yet to be determined
o in an experimental vaccine-challenge study, horses with titres of at least 16 (the lowest level tested)
were protected [2]
o those horses were exposed to 2 million units of HeV (from the 2008 Redlands outbreak) via the nose
and mouth
[1] Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses following vaccination with Equivac® HeV: a field
study. Australian Veterinary Journal 2018; 96: 161–166.
[2] Middleton D, Pallister J, Klein R, et al. Hendra virus vaccine, a One Health approach to protectin horse, human, and environmental health.
Emerging Infectious Diseases 2014; 20: 372—379.

35. § A field study showed that annual boosters are unnecessary in most horses [1]
o the protective threshold for HeV antibodies is yet to be determined
o in an experimental vaccine-challenge study, horses with titres of at least 16 (the lowest level tested)
were protected [2]
o those horses were exposed to 2 million units of HeV (from the 2008 Redlands outbreak) via the nose
and mouth
o the authors noted that “the experimental horses were exposed to considerably higher levels
of HeV than have been recovered from flying foxes...” [2]
[1] Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses following vaccination with Equivac® HeV: a field
study. Australian Veterinary Journal 2018; 96: 161–166.
[2] Middleton D, Pallister J, Klein R, et al. Hendra virus vaccine, a One Health approach to protectin horse, human, and environmental health.
Emerging Infectious Diseases 2014; 20: 372—379.

36. § A field study showed that annual boosters are unnecessary in most horses [1]
o the protective threshold for HeV antibodies is yet to be determined
o in an experimental vaccine-challenge study, horses with titres of at least 16 (the lowest level tested)
were protected [2]
o it is likely that much lower titres are protective in real-world situations
[1] Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses following vaccination with Equivac® HeV: a field
study. Australian Veterinary Journal 2018; 96: 161–166.
[2] Middleton D, Pallister J, Klein R, et al. Hendra virus vaccine, a One Health approach to protectin horse, human, and environmental health.
Emerging Infectious Diseases 2014; 20: 372—379.

37. § A field study showed that annual boosters are unnecessary in most horses [1]
o the protective threshold for HeV antibodies is yet to be determined
o in an experimental vaccine-challenge study, horses with titres of at least 16 (the lowest level tested)
were protected [2]
o it is likely that much lower titres are protective in real-world situations
o this is the only HeV vaccine-challenge study published to date; it didn’t look at titres below 16,
so as these studies are really expensive, we’re unlikely to have definitive answers any time soon
[1] Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses following vaccination with Equivac® HeV: a field
study. Australian Veterinary Journal 2018; 96: 161–166.
[2] Middleton D, Pallister J, Klein R, et al. Hendra virus vaccine, a One Health approach to protectin horse, human, and environmental health.
Emerging Infectious Diseases 2014; 20: 372—379.

38. § What is a titre?

39. § What is a titre?
o the process of measuring the amount of an antibody in a sample (e.g.,blood or serum) involves
titration, which is where ‘titre’ comes from

40. § What is a titre?
o the process of measuring the amount of an antibody in a sample (e.g.,blood or serum) involves
titration, which is where ‘titre’ comes from
o titration in this instance involves a series of 2-fold dilutions or ‘doublings’ (2, 4, 8, 16, 32, 64, and so on)
up to the point at which the antibody is no longer detectable in the sample

41. § What is a titre?
o the process of measuring the amount of an antibody in a sample (e.g.,blood or serum) involves
titration, which is where ‘titre’ comes from
o titration in this instance involves a series of 2-fold dilutions or ‘doublings’ (2, 4, 8, 16, 32, 64, and so on)
up to the point at which the antibody is no longer detectable in the sample
o this particular assay involves neutralisation of live HeV, so it is called a virus-neutralising (VN) or
serum-neutralising (SN) titre

42. § What is a titre?
o the process of measuring the amount of an antibody in a sample (e.g.,blood or serum) involves
titration, which is where ‘titre’ comes from
o titration in this instance involves a series of 2-fold dilutions or ‘doublings’ (2, 4, 8, 16, 32, 64, and so on)
up to the point at which the antibody is no longer detectable in the sample
o the higher the titre, the more dilutions it took before the antibody was no longer detected, so...

43. § What is a titre?
o the process of measuring the amount of an antibody in a sample (e.g.,blood or serum) involves
titration, which is where ‘titre’ comes from
o titration in this instance involves a series of 2-fold dilutions or ‘doublings’ (2, 4, 8, 16, 32, 64, and so on)
up to the point at which the antibody is no longer detectable in the sample
o the higher the titre, the more dilutions it took before the antibody was no longer detected, so...
o the higher the titre, the greater the amount of that antibody there was in the sample; and...

44. § What is a titre?
o the process of measuring the amount of an antibody in a sample (e.g.,blood or serum) involves
titration, which is where ‘titre’ comes from
o titration in this instance involves a series of 2-fold dilutions or ‘doublings’ (2, 4, 8, 16, 32, 64, and so on)
up to the point at which the antibody is no longer detectable in the sample
o the higher the titre, the more dilutions it took before the antibody was no longer detected, so...
o the higher the titre, the greater the amount of that antibody there was in the sample; and...
o the higher the titre, the longer it takes for the titre to drop below a protective level

45. § In the field study, most horses had titres well above the current threshold (16)
after the primary immunisation series (VAC 3 in the graph below)
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

46. § Most horses had titres well above the current threshold after the first dose
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

47. § What this graph doesn’t show is how high the values actually went ...
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

48. The upper limit of the test was 8,192. Any values above this range were recorded as 8,192. How high they actually went, we don't
know. The lower limit was 8. Any values below this range were recorded as 'undetectable', even though the horse may have had
HeV antibodies in its blood and adequate cell-mediated immunity to protect against natural exposure to HeV.
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16
upper and lower limits of
detection

49. "Under laboratory conditions, the horses with serum-neutralising antibody titres (SNT) as low
as 16 were protected from infection. It is possible that protection from field exposure to
virus may also occur in immunised horses with lower (or even undetectable) titres."
Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses following vaccination with Equivac® HeV: a field study.
Australian Veterinary Journal 2018; 96: 161–166.

50. § The first annual booster was unnecessary in most horses
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

51. § Titres remained high after the first annual booster
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

52. § The second annual booster was unnecessary in most horses
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

53. § The second annual booster caused only a modest increase in titre compared
with the increase after the first annual booster
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

54. § We don’t know what happens beyond this study period, but it is likely that the
primary immunisation series ± the 1st annual booster is protective for years
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

55. § This study also shows us that proof of immunity is a reasonable alternative
to proof of vaccination
KEY:
average (mean) values
range (min to max)
range just before vaccine given
threshold of 16

56. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use

57. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
o identify the horses with zero or low titres and vaccinate/booster just those individuals

58. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
o identify the horses with zero or low titres and vaccinate/booster just those individuals
o monitor the progressive decline in titre and booster only when it nears or dips below the accepted
threshold (currently 16, but likely to be less than 8)

59. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
o identify the horses with zero or low titres and vaccinate/booster just those individuals
o monitor the progressive decline in titre and booster only when it nears or dips below the accepted
threshold (currently 16, but likely to be less than 8)
o identify the horses with persistently zero/low titres despite vaccination (poor or non-responders)
and target them for extra HeV precautions (e.g.,strict avoidance of flying foxes and sick horses)

60. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
o identify the horses with zero or low titres and vaccinate/booster just those individuals
o monitor the progressive decline in titre and booster only when it nears or dips below the accepted
threshold (currently 16, but likely to be less than 8)
o identify the horses with persistently zero/low titres despite vaccination (poor or non-responders)
and target them for extra HeV precautions (e.g.,strict avoidance of flying foxes and sick horses)
o these individuals are probably protected by vaccination (which also induces cell-mediated immunity),
despite having low circulating antibody levels, but it’s better to be safe than sorry

61. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
§ It helps horse owners economise without compromising horse and human health

62. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
§ It helps horse owners economise without compromising horse and human health
o testing costs money, but so does unnecessary vaccination

63. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
§ It helps horse owners economise without compromising horse and human health
o testing costs money, but so does unnecessary vaccination
o testing avoids the expense and health consequences of adverse effects (vet visits, medications, time
off from training, missed events, potential for permanent debility/death) when a booster is not needed

64. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
§ It helps horse owners economise without compromising horse and human health
o testing costs money, but so does unnecessary vaccination
o testing avoids the expense and health consequences of adverse effects (vet visits, medications, time
off from training, missed events, potential for permanent debility/death) when a booster is not needed
o in horses at risk for vaccine reaction or other adverse effects, testing allows the vet and horse owner
to weigh the risk vs benefit of vaccinating/boostering using the horse’s own data

65. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
§ It helps horse owners economise without compromising horse and human health
§ Proof of immunity helps vets, farm managers, and event organisers ensure both
the participation and safety of the horses and people in their care

66. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
§ It helps horse owners economise without compromising horse and human health
§ Proof of immunity helps vets, farm managers, and event organisers ensure both
the participation and safety of the horses and people in their care
o mandating annual HeV vaccination excludes many horses and their owners from veterinary services,
boarding/training facilities, and equestrian events

67. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
§ It helps horse owners economise without compromising horse and human health
§ Proof of immunity helps vets, farm managers, and event organisers ensure both
the participation and safety of the horses and people in their care
o mandating annual HeV vaccination excludes many horses and their owners from veterinary services,
boarding/training facilities, and equestrian events
o mandating annual HeV vaccination is not the only way to ensure that horses and people are protected
from HeV

68. § Testing for the presence of HeV antibodies in the horse’s blood allows us to tailor
HeV vaccination for optimal safety, effectiveness, and vaccine use
§ It helps horse owners economise without compromising horse and human health
§ Proof of immunity helps vets, farm managers, and event organisers ensure both
the participation and safety of the horses and people in their care
o mandating annual HeV vaccination excludes many horses and their owners from veterinary services,
boarding/training facilities, and equestrian events
o mandating annual HeV vaccination is not the only way to ensure that horses and people are protected
from HeV
o mandating annual HeV vaccination is unjustified in horses with demonstrable immunity against HeV

69. § Currently, there are two types of HeV antibody tests:
1. Virus neutralisation (VN) test
2. Enzyme-linked immunosorbent assay (ELISA)

70. § The VN test result is reported as a numeric value (a titre)

71. § The VN test result is reported as a numeric value (a titre)
o a titre of 16 or above is currently considered protective against HeV

72. § The VN test result is reported as a numeric value (a titre)
o a titre of 16 or above is currently considered protective against HeV
o it is likely that much lower titres (4 or lower) are protective, as antibody production is rapidly
increased in the face of challenge (natural infection or vaccination) [1]
[1] Colling A, Lunt R, Bergfeld J, et al. A network approach for provisional assay of a Hendra virus antibody ELISA: test validation
with low sample numbers from infected horses. Journal of Veterinary Diagnostic Investigation 2018; 30: 362–369.

73. § The VN test result is reported as a numeric value (a titre)
o a titre of 16 or above is currently considered protective against HeV
o it is likely that much lower titres (4 or lower) are protective, as antibody production is rapidly
increased in the face of challenge (natural infection or vaccination) [1]
o for example, in one horse the titre increased from less than 2 to 64 within 1 week
[1] Colling A, Lunt R, Bergfeld J, et al. A network approach for provisional assay of a Hendra virus antibody ELISA: test validation
with low sample numbers from infected horses. Journal of Veterinary Diagnostic Investigation 2018; 30: 362–369.

74. § The VN test result is reported as a numeric value (a titre)
o a titre of 16 or above is currently considered protective against HeV
o it is likely that much lower titres (4 or lower) are protective, as antibody production is rapidly
increased in the face of challenge (natural infection or vaccination) [1]
o for example, in one horse the titre increased from less than 2 to 64 within 1 week
o the incubation period for natural HeV infection ranges from 5 to 16 days, so horses with pre-existing
immunity have enough time to mount a robust antibody response [2]
[1] Colling A, Lunt R, Bergfeld J, et al. A network approach for provisional assay of a Hendra virus antibody ELISA: test validation
with low sample numbers from infected horses. Journal of Veterinary Diagnostic Investigation 2018; 30: 362–369.
[2] Broder CC, Weir DL, Reid PA. Hendra virus and Nipah virus animal vaccines. Vaccine 2016; 34: 3525–3534.

75. § The VN test result is reported as a numeric value (a titre)
o a titre of 16 or above is currently considered protective against HeV
o it is likely that much lower titres (4 or lower) are protective, as antibody production is rapidly
increased in the face of challenge (natural infection or vaccination) [1]
o in addition, antibodies are only one component of the immune response to viral challenge

76. § The VN test result is reported as a numeric value (a titre)
o a titre of 16 or above is currently considered protective against HeV
o it is likely that much lower titres (4 or lower) are protective, as antibody production is rapidly
increased in the face of challenge (natural infection or vaccination) [1]
o in addition, antibodies are only one component of the immune response to viral challenge
o titre testing every 1–2 years should be adequate for monitoring (the higher the titre, the longer
it will take to drop)

77. § The VN test result is reported as a numeric value (a titre)
o a titre of 16 or above is currently considered protective against HeV
o it is likely that much lower titres (4 or lower) are protective, as antibody production is rapidly
increased in the face of challenge (natural infection or vaccination) [1]
o in addition, antibodies are only one component of the immune response to viral challenge
o titre testing every 1–2 years should be adequate for monitoring (the higher the titre, the longer
it will take to drop)
o HOWEVER, this test is currently very expensive ($367.40*)
* CSIRO Australian Animal Health Laboratory; https://aahl.csiro.au/info/companion_and_equine_testing.aspx [March 2021]

78. § The ELISA result is reported only as ‘positive’,‘negative’, or ‘inconclusive’

79. § The ELISA result is reported only as ‘positive’,‘negative’, or ‘inconclusive’
o the ELISA is more affordable ($111.10*) but it does not provide a titre
* CSIRO Australian Animal Health Laboratory; https://aahl.csiro.au/info/companion_and_equine_testing.aspx [March 2021]

80. § The ELISA result is reported only as ‘positive’,‘negative’, or ‘inconclusive’
o the ELISA is more affordable ($111.10*) but it does not provide a titre
o a positive ELISA confirms the presence of HeV-specific, virus-neutralising antibodies

81. § The ELISA result is reported only as ‘positive’,‘negative’, or ‘inconclusive’
o the ELISA is more affordable ($111.10*) but it does not provide a titre
o a positive ELISA confirms the presence of HeV-specific, virus-neutralising antibodies
o in the study that validated the CSIRO’s ELISA (which targets the HeV and vaccine G glycoprotein),
the breakpoint between ‘inconclusive’ and ‘positive’ was a VN titre of 2.4 [1]
[1] Colling A, Lunt R, Bergfeld J, et al. A network approach for provisional assay of a Hendra virus antibody ELISA: test validation
with low sample numbers from infected horses. Journal of Veterinary Diagnostic Investigation 2018; 30: 362–369.

82. § The ELISA result is reported only as ‘positive’,‘negative’, or ‘inconclusive’
o the ELISA is more affordable ($111.10*) but it does not provide a titre
o a positive ELISA confirms the presence of HeV-specific, virus-neutralising antibodies
o any amount of HeV antibody indicates prior exposure (natural or vaccinal) and the development
of an appropriate immune response, which includes cell memory

83. § The ELISA result is reported only as ‘positive’,‘negative’, or ‘inconclusive’
o the ELISA is more affordable ($111.10*) but it does not provide a titre
o a positive ELISA confirms the presence of HeV-specific, virus-neutralising antibodies
o any amount of HeV antibody indicates prior exposure (natural or vaccinal) and the development
of an appropriate immune response, which includes cell memory
o the exception may be foals who received antibodies in their mother’s colostrum (‘passive transfer’
of temporary immunity); they have not yet developed their own immune response to HeV

84. § The ELISA result is reported only as ‘positive’,‘negative’, or ‘inconclusive’
o the ELISA is more affordable ($111.10*) but it does not provide a titre
o a positive ELISA confirms the presence of HeV-specific, virus-neutralising antibodies
o any amount of HeV antibody indicates prior exposure (natural or vaccinal) and the development
of an appropriate immune response, which includes cell memory
o horses with a positive ELISA are likely to be adequately protected for their own sake and unlikely
to pose a risk to other horses or to people

85. § The ELISA result is reported only as ‘positive’,‘negative’, or ‘inconclusive’
o the ELISA is more affordable ($111.10*) but it does not provide a titre
o a positive ELISA confirms the presence of HeV-specific, virus-neutralising antibodies
o any amount of HeV antibody indicates prior exposure (natural or vaccinal) and the development
of an appropriate immune response, which includes cell memory
o horses with a positive ELISA are likely to be adequately protected for their own sake and unlikely
to pose a risk to other horses or to people
o horses with an inconclusive or negative ELISA should be titre-tested or vaccinated/boostered,
unless there is a medical reason not to...

86. Vaccination is not a benign procedure; it is deliberately provocative.

87. Vaccination is not a benign procedure; it is deliberately provocative.
For this reason, vaccination is only for healthy animals.

88. Vaccination is not a benign procedure; it is deliberately provocative.
For this reason, vaccination is only for healthy animals.
Sick or debilitated animals should not be vaccinated.

89. Vaccination is not a benign procedure; it is deliberately provocative.
For this reason, vaccination is only for healthy animals.
Sick or debilitated animals should not be vaccinated.
Animals with a history of allergy, hypersensitivity, or vaccine-associated disease
should be vaccinated with great care, at an extended interval, or not at all.

90. While we’re at it...
In the experimental vaccine-challenge study, 3 horses received half the dose used in the commercial
vaccine (50 µg instead of 100 µg of the HeV G glycoprotein). The lowest pre-challenge antibody titre
in this group was 128. They were all protected against viral challenge. [1]
This may be another viable option for horses with a history of vaccine-associated disease.
[1] Middleton D, Pallister J, Klein R, et al. Hendra virus vaccine, a One Health approach to protectin horse, human, and environmental health.
Emerging Infectious Diseases 2014; 20: 372—379.

91. In Conclusion:

92. In Conclusion:
§ Proof of immunity is a reasonable alternative to proof of vaccination against HeV

93. In Conclusion:
§ Proof of immunity is a reasonable alternative to proof of vaccination against HeV
§ Either of the currently available Hendra antibody tests (VN titre or ELISA) show
whether the horse has antibodies against HeV in its bloodstream

94. In Conclusion:
§ Proof of immunity is a reasonable alternative to proof of vaccination against HeV
§ Either of the currently available Hendra antibody tests (VN titre or ELISA) show
whether the horse has antibodies against HeV in its bloodstream
§ A positive result serves as proof of immunity (circulating antibodies) at that time;
how often the test should be repeated remains to be determined

95. In Conclusion:
§ Proof of immunity is a reasonable alternative to proof of vaccination against HeV
§ Either of the currently available Hendra antibody tests (VN titre or ELISA) show
whether the horse has antibodies against HeV in its bloodstream
§ A positive result serves as proof of immunity (circulating antibodies) at that time;
how often the test should be repeated remains to be determined
o based on available data, it would be reasonable to retest every 1–2 years, depending on the
circumstances, previous results, and level of HeV risk