| Summary: | <p>Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has led to a global pandemic since its first identification in December 2019. In response to the pandemic, multiple vaccines have been developed, which showed good initial effectiveness, but different variants of concern caused by genetic mutations kept emerging and hampered vaccine protection. Assessment of the immunity acquired from both natural infection and vaccinations is important to optimize vaccination strategies and pandemic management.</p>
<p>This thesis aims to examine the nature and duration of antibody responses after SARS-CoV-2 infection and vaccination, analyse the association between antibody levels and protection (i.e. correlates of protection), and estimate the duration of protection following infection or vaccination in the UK general population.</p>
<p>Using data from the UK’s COVID-19 Infection Survey (CIS), which is the largest household survey of COVID-19 infections and antibodies in the UK, I conducted five studies examining the antibody responses after Pre-Alpha and Alpha infections, after the first vaccination, after the second vaccination, and after the booster vaccination or breakthrough infection. Using latent class mixed models, I found that around 24% of people did not generate positive antibody response after infection, and 5-6% did not respond to the first vaccination, who were older and with more long-term health conditions. Using Bayesian linear mixed models, I estimated that the mRNA vaccines (BNT162b2 and mRNA-1273) elicited much higher antibody levels than the adenovirus vaccine (ChAdOx1) and natural infection, but antibodies after infection lasted longer. I also found that people with prior infection had much stronger antibody responses after the first and second vaccination. Using generalised additive models, I found that higher antibody levels were associated with a higher level of protection against (re)infections in the Delta and Omicron BA.4/5 epochs, and a hybrid immunity from both vaccination and infection led to much higher protection given the same antibody levels. Combining these results, I concluded that vaccine protection was relatively short-lived especially against Omicron infections, while natural infection induced longer protection.</p>
<p>This work has contributed to the understanding of immunogenicity and associated protection of SARS-CoV-2 natural infection and vaccination in the UK general population, which provides evidence to the UK’s national vaccination strategies and policy making over the pandemic.</p>
|
|---|