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COVID-19 vaccine update from the latest Lancet article (4 Nov 2020)
[1] As the efficacy decreases, the population that needs to be vaccinated increases.
(a) For a vaccine with 100% efficacy that gives life-long protection, herd immunity required to block transmission is about 60–72%. (Considering that most countries had an early R0 of 2·5 to 3·5)
(b) Vaccine efficacies desirable to protect against infection > 80%. Now, if you add the proportional vaccine efficacy and assume 80% efficacy then the herd immunity required becomes 75–90%
(c) Any efficacy lower than this will require the entire population to be immunized.
[2] What we know till now:
(a) Vaccine efficacies to protect against infection above 80% are desirable but duration of protection will remain uncertain for a number of years post licensure of COVID-19 vaccines.
(b) Antibody titres in recovered people might be declining, but antibodies are only one part of immune response & acquired immunity to reinfection or disease prevention when reinfected.
(c) Immunity to SARS-CoV-2 might be short lived, perhaps 12–18 months in duration. (Based on data of ‘other coronaviruses’)
(d) Whether past infection will prevent severe COVID-19 on re-exposure to SARS-CoV-2 is not known at present.
[3] Vaccine Timeline:
(a) Out of 45 vaccines in clinical trials, 10 are in phase 3 trials. If Phase 3 results come in before 2020 ends and the results are satisfactory, wide-scale deployment of COVID-19 vaccines may be possible in the second half of 2021.
(b) Phase 3 trials (results on efficacy) will tell us the amount of vaccine required for a defined population
(c) Phase 4 trials will tell us the duration of protection against infection as well as severe disease.
(d) Vaccine delivery will probably scale up gradually as manufacturing capabilities develop over 12–24 months post licensure.
(e) Impact of vaccination on #sarscov2 transmission will start slowly and build up over a few years to reach target coverage levels.
[4] Things become complicated if we assume that immunity is short lived.
(a) Population % that must be vaccinated in year 1 is much larger than the percentage that must be vaccinated once the system has stabilised after a few years
(b) Reason: Most of the population will be susceptible as mass immunisation starts, but after a few years, hopefully, a high proportion will be immunised such that effective herd immunity is created.
(c) If efficacy is satisfactory (>80%) but duration of protection is short (1–2 years) – and this assumption comes true then a large proportion of the total population would need to be vaccinated if there is to be any chance of getting herd immunity to block the continued transmission of SARS-CoV-2.
(d) In countries that do not reach high vaccine coverage SARS-CoV-2 will become endemic but at a low level with seasonal peaks in winter etc.
Link: The Lancet 4 Nov 2020
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)32318-7/fulltext
