Early 2022 breakthrough infection sera from India target the conserved cryptic class 5 epitope to counteract immune escape by SARS-CoV-2 variants

During the COVID-19 pandemic, the vast majority of epitope mapping studies have focused on sera from mRNA-vaccinated populations from high-income countries. In contrast, here we report an analysis of 164 serum samples isolated from breakthrough infection patients in India during early 2022 who received two doses of the ChAdOx viral vector vaccine. Sera were screened for neutralization breadth against wildtype, Kappa, Delta, and Omicron BA.1 viruses. Three sera with the highest neutralization breadth and potency were selected for epitope mapping using charged scanning mutagenesis coupled to yeast surface display and Next Generation Sequencing. All of these sera primarily targeted the recently identified class 5 cryptic epitope along with the class 1 and class 4 epitopes, targeted to a lesser extent. The class 5 epitope is completely conserved across all SARS-CoV-2 variants and for the majority of the Sarbecoviruses. In line with these observations, a major fraction of the serum samples, including the selected three, show broad neutralizing activity against recent Omicron subvariants including XBB.1.5. This is in contrast with the results obtained with the sera from individuals receiving multiple doses of original and updated mRNA vaccines, where impaired neutralization of XBB and later variant of concerns was observed. Our study demonstrates that two doses of the ChAdOx vaccine in a highly exposed population was sufficient to drive substantial neutralization breadth against emerging and upcoming variants of concern, thus paves the path towards the development of future vaccine candidates.