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Critical weaknesses in shielding strategies for COVID-19
- 1. Critical weaknesses in shielding strategies for COVID-19 Centre for Mathematical Biology Christian (Kit) Yates www.kityates.com @Kit_Yates_Maths c.yates@bath.ac.uk University of Bath 16/03/23
- 2. Herd Immunity through natural infection… Vallance (13th March): “Our aim is to try to reduce the peak, broaden the peak, not suppress it completely; also, because the vast majority of people get a mild illness, to build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission, at the same time we protect those who are most vulnerable to it.”
- 3. … is a bad idea “But herd immunity acquired by natural infection has several disadvantages as a means of COVID-19 control” (1) The build-up of herd immunity would be associated with a high burden of illness and death; (2) While infection is spreading through a population, it is not yet clear that the most vulnerable can be protected from severe and fatal COVID-19; and (3) It may not be possible to achieve herd immunity by natural infection if protection against reinfection is partial and transient. The preferred route to herd immunity is not through natural infection but by vaccination.” From: “Herd immunity in the epidemiology and control of COVID-19” https://royalsociety.org/-/media/policy/projects/set-c/set-c-herd-immunity.pdf
- 5. The Great Barrington Declaration
- 6. The Great Barrington Declaration “As immunity builds in the population, the risk of infection to all – including the vulnerable – falls. We know that all populations will eventually reach herd immunity – i.e. the point at which the rate of new infections is stable – and that this can be assisted by (but is not dependent upon) a vaccine.” “The most compassionate approach that balances the risks and benefits of reaching herd immunity, is to allow those who are at minimal risk of death to live their lives normally to build up immunity to the virus through natural infection, while better protecting those who are at highest risk. We call this Focused Protection.”
- 7. No mitigations Low risk High risk
- 8. Lockdown Low risk High risk
- 9. Focussed protection - theory Low risk High risk
- 10. Focussed protection - practice
- 11. Low High Low Low Low High High High Multi-risk households Non-residential care High Misdiagnosed or undiagnosed risk Imperfect Shielding
- 12. Uneven distribution of immunity Low risk High risk Immune
- 13. • Inability to effectively shield higher-risk individuals (imperfect shielding) • Uneven distribution of immunity • People may voluntarily change their behaviour (reduced contact) • Inability to identify many people at higher-risk (or to self-identify risk) • If shielding fails then it may be difficult to bring cases under control • Unnecessary mortality and morbidity (such as Long covid) • Potential for overwhelming healthcare capacity • Unknown duration or efficacy of naturally acquired immunity • Increased mutation supply, potentially leading to new variants (which evade immunity) • Ethical implications for the prolonged isolation of higher-risk individuals with reduced access to health and social care Potential weaknesses with shielding strategies
- 14. • Inability to effectively shield higher-risk individuals (imperfect shielding) • Uneven distribution of immunity • People may voluntarily change their behaviour (reduced contact) • Inability to identify many people at higher-risk (or to self-identify risk) • If shielding fails then it may be difficult to bring cases under control • Unnecessary mortality and morbidity (such as Long covid) • Potential for overwhelming healthcare capacity • Unknown duration or efficacy of naturally acquired immunity • Increased mutation supply, potentially leading to new variants (which evade immunity) • Ethical implications for the prolonged isolation of higher-risk individuals with reduced access to health and social care Potential weaknesses with shielding strategies
Editor's Notes
- Whether something takes off and grows exponentially or dies off exponentially depends on one number – The basic reproduction number. R0
- If a disease has an R0 of less than 1, then the infection will die out quickly as each infectious person passes on the disease, on average, to less than one other individual. The outbreak cannot sustain its own spread. If the R0 is larger than 1 then the outbreak will grow exponentially. Take a disease like SARS, for example, with a basic reproduction number of 2. The first person with the disease is Patient Zero. They spread the disease to two others, who each spread the disease to two others and thence to two others each. If the spread was able to continue like this, ten generations down the chain of progression, over 1000 people would be infected. Ten steps further on, the toll would rise to over one million. In practice, just as with the spread of a viral idea, the expansion of a pyramid scheme, the growth of a bacterial colony or the proliferation of a population, the exponential growth predicted by the basic reproduction number is rarely sustained beyond a few generations. Outbreaks eventually peak and then decline due to the decreasing frequency of infected–susceptible contacts.
- Whether something takes off and grows exponentially or dies off exponentially depends on one number – The basic reproduction number. R0
- Whether something takes off and grows exponentially or dies off exponentially depends on one number – The basic reproduction number. R0
- Whether something takes off and grows exponentially or dies off exponentially depends on one number – The basic reproduction number. R0
- Don’t forget to mention there are lots of other strategies. Isolation/Quarantin. Test and trace. Ventilation. (eventually vaccination)