Immunisation, asymptomatic infection, herd immunity and the new variants of COVID-19

ObjectivesIs "herd immunity" to COVID-19 a realistic outcome of any immunisation programme with the two main vaccines currently licenced in the UK (Pfizer vaccine BNT162b2 and Astra Zeneca/Oxford vaccine ChAdOx1-S)? More formally, can these vaccines achieve a sufficient level of population immunity to reduce R, the reproduction number of the infection, to below one in the absence of any non-pharmaceutical interventions? DesignThe study uses simple mathematical models of the transmission of COVID-19 infection from primary to secondary cases parameterised using data on virus transmission and vaccine efficacy from the literature and the regulatory approval process for the vaccines. ResultsIn the regulatory approval documents, the efficacy of the Pfizer vaccine is estimated at 0.948 (that for the Moderna vaccine is similar). Efficacy for the Oxford vaccine against primary symptomatic illness is estimated as 0.704, based on pooling of data from two dose regimes. For values of R0 similar to those reported during the first months of the pandemic, the simplest analysis implies that reducing the value of R below 1 would require 69% and 93% of the population to be vaccinated with the Pfizer and Oxford vaccine respectively (or achieve a comparable level of immunity through natural infection). However, the new variant of COVID-19 (Lineage B.1.1.7, named Variant of Concern VOC-202012/01) is reported to have an R-value 1.56 (0.92 to 2.28) times higher than the original strain. Vaccinating the entire population with the Oxford vaccine would only reduce the R value to 1.325 while the Pfizer vaccine would require 82% of the population to be vaccinated to control the spread of the new variant. The Oxford vaccine reduces the incidence of serious illness to a greater extent than it reduces symptomatic illness. But its efficacy against the incidence of asymptomatic infections is lower, reducing its efficacy against all infection from 0.704 to 0.525 for the pooled data. Although asymptomatics are less infectious, including them in our calculations still raises R values by 20% or more, from 1.33 to 1.6 for the new variant with 100% vaccination. Neither vaccine is licenced for use in children, and when this is taken into account, this R value rises by a further 37% to 2.2 if the whole adult population is vaccinated. Even the more effective mRNA vaccines may allow the pandemic to persist via transmission amongst children, as current authorisations only allow their use on adults. In the absence of vaccination, R will reduce to 1 when 89% of the population has acquired immunity as a result of previous infection with COVID-19. ConclusionsAll currently licensed vaccines provide substantial protection against serious illness to vaccinated individuals themselves. But the Oxford vaccine appears to have relatively low efficacy against asymptomatic infections. Although no comparable data from human trials are available for the mRNA vaccines, non-human primate studies suggest they are better at preventing nasal shedding and so transmission. Herd immunity to COVID-19 will be very difficult to achieve, especially so for the less effective vaccine. The possibility of transmission from vaccinated but infected individuals to vulnerable unvaccinated individuals is of serious concern. There is a strong case for preferring the more effective mRNA vaccines for health and social care workers and those who have contact with large numbers of vulnerable others.