SARS-CoV-2 Omicron variant BA. 2 neutralisation in sera of people with Comirnaty or CoronaVac vaccination, infection or breakthrough infection, Hong Kong, 2020 to 2022

Background: Omicron subvariant BA.2 circulation is rapidly increasing globally. Aim: We evaluated the neutralising antibody response from vaccination or prior SARS-CoV-2 infection against symptomatic infection by BA.2 or other variants. Methods: Using 50% plaque reduction neutralisation tests (PRNT50), we assessed neutralising antibody titres to BA. 2, wild type (WT) SARS- CoV-2 and other variants in Comirnaty or CoronaVac vaccinees, with or without prior WT-SARS-CoV-2 infection. Titres were also measured for non- vaccinees convalescing from a WT-SARS- CoV-2 infection. Neutralising antibodies in BA.2 and BA.1 breakthrough infections and in BA. 2 infections affecting non- vaccinees were additionally studied. Results: In vaccinees or prior WT-SARS-CoV-2-infected people, BA.2 and BA.1 PRNT 50 titres were comparable but significantly (p < 10(-5)) lower than WT. In each group of 20 vaccinees with (i) three-doses of Comirnaty, (ii) two CoronaVac followed by one Comirnaty dose, or (iii) one dose of either vaccine after a WT-SARS-CoV-2 infection, = 19 individuals developed detectable (PRNT50 titre <= 10) antibodies to BA.2, while only 15 of 20 vaccinated with three doses of CoronaVac did. Comirnaty vaccination elicited higher titres to BA.2 than CoronaVac. In people convalescing from a WT-SARS-CoV-2 infection, a single vaccine dose induced higher BA.2 titres than three Comirnaty (p = 0.02) or CoronaVac (p = 0.00001) doses in infection-naive individuals. BA.2 infections in previously uninfected and unvaccinated individuals elicited low (PRNT50 titre <= 80) responses with little cross-neutralisation of other variants. However, vaccinees with BA.1 or BA.2 breakthrough infections had broad cross- neutralising antibodies to WT viruses, and BA.1, BA.2, Beta and Delta variants. Conclusions: Existing vaccines can be of help against the BA.2 subvariant.