Immunisation, asymptomatic infection, herd immunity and the new variants of COVID-19 (original) (raw)

Objectives Is 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? Design The 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. Results In 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...