Project #61:
Estimating COVID-19 vaccine effectiveness in early recipients of the vaccine under the UK national vaccination programme

The COVID-19 vaccines were developed and rigorously evaluated in randomized trials within a year of the first reports of COVID-19 in Wuhan, China. However, many questions about vaccine effectiveness were not answered by randomized trials, for example protection for sick or elderly people, or against severe outcomes such as hospitalisation. The UK started its general population COVID-19 vaccination programme in early December 2020, prioritising vaccination in the most vulnerable people, in particular elderly people, who were not well-represented in trials.

We assessed vaccine effectiveness amongst >2.6 million people aged over 70 in England, using linked routine clinical data from the OpenSAFELY TPP database. We looked at the vaccine’s ability to reduce infection, hospitalisation, and death compared with those who were not vaccinated. We studied both the Pfizer-BioNTech and the Oxford-AstraZeneca vaccines.

We found that the vaccines were highly effective. However, we also found evidence of biases in these estimates. For example, the estimated protective effect of vaccination was very high in the first two weeks after vaccination, yet we know from trials that immunity does not develop until after the first week.

We discuss the challenges of estimated vaccine effectiveness in observational data, using our study as an exemplar.