2. Vaccine effectiveness means that the vaccine has demonstrated its ability to protect under real-life conditions. It
reflects the clinical efficacy of the vaccine, the characteristics of the individual vaccinated (age and immune status) and
programme errors (cold chain and vaccine preparation and administration technique).
• Vaccine effectiveness can be measured if there is doubt about the impact of vaccination.
• The vaccination coverage indicator is essential to evaluating the effectiveness of the immunisation strategy.
When to calculate vaccine effectiveness?
The effectiveness of the vaccine under real conditions is calculated in the following situations:
• when an outbreak occurs in a correctly-vaccinated population (very high vaccination coverage);
• when there is no reduction in measles incidence despite high vaccination coverage;
• when there are a significant percentage of vaccinated people among measles cases;
• when a new vaccine is introduced.
Principle :
The percentage reduction in the attack rate is calculated for those vaccinated versus those not vaccinated. This yields
the preventive fraction in the vaccinated group. The formula is:
NVAR − VAR
VE (in %) = ––––––––––––––
NVAR
Vaccine Effectiveness
3. VE Vaccine effectiveness
NVAR Attack rate among the unvaccinated
VAR Attack rate among the vaccinated
The study should be done quickly (ideally in about ten days).
4. Measurement Methods of Vaccine Effectiveness
Three methods are currently used: the rapid estimation method, cohort studies and case control
studies. The last two require a specialised epidemiology team.
Rapid estimation Method
This is the easiest first-line method for
identifying a problem with vaccine
effectiveness and deciding whether to do
a more in-depth evaluation.
1. Cohort study
This complex method allows
comparison of the attack rates
among the vaccinated (VAR)
and the unvaccinated (NVAR)
and calculation of the relative
risk
2.
Case-control study
This study compares the vaccination
status of a sample of cases and controls.
3.
5. • If possible, use vaccination coverage data from surveys for the calculation. Otherwise, use the estimated
vaccination coverage (administrative).
• The effectiveness is estimated based on the percentage of the population vaccinated (measles vaccination
coverage) and the percentage of vaccinated among the measles cases:
PPV − PCV
VE (en %) = ––––––––––––––––––
PPV x (1 – PCV)
The curves generated by this equation (nomogram) allow rapid evaluation of vaccine effectiveness.
Rapid Estimation Test
VE Vaccine effectiveness
PCV Percentage of cases vaccinated
PPV Percentage of the population vaccinated
6. The curves generated by this equation (nomogram)
allow rapid evaluation of vaccine effectiveness.
7. • If the result is ≥ 80%, one can assume that there is no vaccine effectiveness problem.
• If, on the other hand, the calculated vaccine effectiveness is < 80%, evaluation with a more accurate method
(cohort or case-control study) is needed.
Note, however, that when coverage is low this method yields a biased estimate of vaccine effectiveness.
Example: a vaccination campaign was conducted 9 months ago. An vaccination coverage survey showed that 85%
of the target population was vaccinated (PPV = 85%). Some measles cases have been reported; half of them
were vaccinated during the campaign (PCV = 50%).
EV = [0.85 - 0.50] / 0.8 x (1 - 0.5) = [0.35] / (0.425) = 82%
• Drawing a straight vertical line from 85% on the x-axis and a straight horizontal line from 50% on the y-axis, the
two lines intersect in the vaccine effectiveness region between the 0.8 and the 0.9 curves, consistent with the
value obtained by calculation.
8. This complex method allows comparison of the attack rates among the vaccinated (VAR) and the
unvaccinated (NVAR) and calculation of the relative risk, that is, the difference in the risk of developing the
disease when vaccinated.
The formula is as follows:
VAR
Relative risk = –––––––––
NVAR
or
VE (%) = (1 – relative risk) x 100
Example: Measles attack rate among the vaccinated (VAR) = 1%
Measles attack rate among the unvaccinated (NVAR) = 7%
Relative risk = 0.01/0.07 = 0.14 or VE = 1 – 0.14 = 86%
Cohort Study
9. • This study compares the vaccination status of a sample of cases and controls. It does not study the at-risk
population. The cases represent a sampling fraction of all measles cases, and the controls represent a
sampling fraction of the population that did not get the disease.
For each case, at least one matched control (in terms of age, gender, place of residence, initial health status,
etc.) is chosen at random.
• Knowing the vaccination status of the cases and controls allows estimation of the relative risk (comparing the
attack rate for the vaccinated to the attack rate for the unvaccinated) by measuring the odds ratio (OR).
Case control study
