Infection- or AZD1222 vaccine-mediated immunity reduces SARS-CoV-2 transmission but increases competitiveness of Omicron in hamsters

To understand the impact of pre-existing immunity on SARS-CoV-2 airborne transmission efficiency, we performed a transmission chain experiment using naive, intranasally or intramuscularly AZD1222 vaccinated, and previously infected hamsters. Vaccination and previous infection reduced disease and virus replication. A clear gradient in reduction of transmission blockage was observed: Intramuscular vaccination displayed moderate transmission reduction over three airborne chains (approx. 60%), whereas intranasal vaccination and previous infection displayed a >80% reduction in transmission. Delta outcompeted Omicron after dual infection within and between hosts in naive, vaccinated, and previously infected transmission chains, yet an increase in Omicron competitiveness was observed in groups with pre-existing immunity against Delta. This correlated with an increase in the strength of the humoral response against Delta, with the strongest response seen in previously infected animals. These data highlight the continuous need to improve vaccination strategies and address the additional evolutionary pressure pre-existing immunity may exert on SARS-CoV-2.