The Pfizer vaccine showed 95% efficacy against the development of COVID-19. If the vaccine could also prevent transmission of the virus by 95%, then assuming basic reproduction number (R0) of SARS-CoV-2 as 3, around 71% [(1-1/R0)x(100/95)] would need to be vaccinated to achieve herd immunity. The remaining 29% would then be protected from SARS-CoV-2 infection even without receiving the vaccine. For the Oxford-AstraZeneca vaccine (70% efficacy), the vaccination coverage threshold would be 96%. This means the Oxford vaccine would allow very little room for the people to remain unvaccinated.
But there is a major error in having a discussion similar to the one above. The ability to prevent transmission of the virus from an infected person to a healthy individual was not a trial endpoint that was assessed for Pfizer vaccine. So we are yet to consider 95% efficacy for calculating vaccination threshold for herd immunity. For Oxford/AstraZeneca vaccine, there is early indication that their vaccine prevented transmission of the virus to some extent, as their was reduction in asymptomatic infection among the vaccine recipients. But again, they would need to provide results from their follow-up data, from larger samples.
Till then, we would need to be cautious regarding calculating vaccination threshold against COVID-19 for achieving herd immunity and interpreting them.
Resources used for this presentation:
The false promise of herd immunity for COVID-19 (Nature):
Pfizer and BioNTech Conclude Phase 3 Study of COVID-19 Vaccine Candidate, Meeting All Primary Efficacy Endpoints (BioNTech):
This video has been created by Dr Kazi Mizanur Rahman. Dr Rahman is a medical doctor and epidemiologist, and work directly for COVID-19 in both research and program spaces.
Dr Rahman did MS in Epidemiology from Harvard University and PhD from the National Centre for Epidemiology and Population Health at the Australian National University. He is a medical graduate from Dhaka Medical College, University of Dhaka. Dr Rahman worked for more than a decade at the International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b) as a public health researcher. He taught at James P Grant School of Public Health, BRAC University for five years. In Australia, Dr Rahman has been working as an epidemiologist both in the academia and programs with a focus on protection and promotion of health at the population level.
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