The importance of time post-vaccination in determining the decrease in vaccine efficacy against SARS-CoV-2 variants of concern

With the development of high-efficacy vaccines against SARS-CoV-2, an urgent open question is whether currently available vaccines protect with similar efficacy against infection with SARS-CoV-2 variants of concern (VOC). Recent reports quantifying the extent by which VOC can evade vaccine immunity resulted in a range of estimates for the same VOC, which makes them difficult to interpret. One possible explanation for the discrepancies between different studies is an inconsistency in terms of the time post-vaccination of the sampled population. Here we present a model based on the observed correlation between antibody neutralization levels and vaccine efficacy, which demonstrates the impact of time post-vaccination on the comparison of the vaccine efficacy for VOC versus non-VOC infections. Our model predicts and exemplifies several possible consequences for vaccine efficacy in VOC infections: 1) a delay in the onset of vaccine efficacy against VOC; 2) a transient increase in susceptibility to breakthrough infection with VOC compared to non-VOC as a function of time after vaccination. We review preliminary data indicating that such phenomena are observed in studies of the B.1.1.7 and B.1.351 variants. We find that ignoring the strong dependence on the time post-vaccination can lead to contradictory reports of relative efficacy against VOC versus non-VOC, with implications on mitigation strategies against VOC and the design of vaccine efficacy studies.