Omicron breakthrough infections in vaccinated or previously infected hamsters

The second and third years of the SARS-CoV-2 pandemic have been marked by the repeated emergence and replacement of ‘variants’ with genetic and phenotypic distance from the ancestral strains, the most recent examples being Delta and Omicron. Here we describe a hamster contact exposure challenge model to assess protection conferred by vaccination or prior infection against re-infection. We found that 2-doses of self-amplifying RNA vaccine based on the ancestral spike ameliorated weight loss following Delta infection and decreased viral loads, but had minimal effect on Omicron/BA.1 infection. Prior infection with ancestral or Alpha variant was partially protective against Omicron/BA.1 infection, whereas all animals previously infected with Delta and exposed to Omicron became infected, although shed less virus. We further tested whether prior infection with Omicron/BA.1 protected from re-infection with Delta or Omicron/BA.2. Omicron/BA.1 was protective against Omicron/BA.2, but not Delta reinfection, again showing Delta and Omicron have a very large antigenic distance. Indeed, cross-neutralisation assays with human antisera from otherwise immunonaïve individuals (unvaccinated and no known prior infection), confirmed a large antigenic distance between Delta and Omicron. Prior vaccination followed by Omicron or Delta breakthrough infection led to a higher degree of cross-reactivity to all tested variants. To conclude, cohorts whose only immune experience of COVID is Omicron/BA.1 infection may be particularly vulnerable to future circulation of Delta or Delta-like derivatives. In contrast, repeated exposure to antigenically distinct spikes, via infection and or vaccination drives a more cross-reactive immune response, both in hamsters and people.