LAUSR

The Omicron variant is rapidly becoming the dominant SARS-CoV-2 virus circulating globally. It is important to define reductions in virus neutralizing activity in serum of convalescent or vaccinated individuals to understand potential loss of protection against infection by Omicron. We have previously established that a 50% plaque reduction neutralization (PRNT50) antibody titre ≥25.6 in our live virus assay corresponded to the threshold for 50% protection from infection against wild-type (WT) SARS-CoV-2. Here we show markedly reduced serum antibody titres against the Omicron variant (geometric mean titre (GMT) <10) as compared to wild-type virus 3-5 weeks after two doses of BNT162b2 (GMT 218.8) or CoronaVac vaccines (GMT 32.5). A BNT162b2 booster dose elicited Omicron PRNT50 titres ≥25.6 in 88% of individuals (22 of 25) who previously received 2 doses of BNT162b2 and 80% of individuals (24 of 30) who previously received CoronaVac. However, few (3%) previously infected individuals (1 of 30) or those vaccinated with three doses of CoronaVac (1 of 30) met this threshold. Our findings suggest that countries primarily using CoronaVac vaccines should consider mRNA vaccine boosters in response to the spread of Omicron. Studies evaluating the effectiveness of different vaccines against the Omicron variant are urgently needed.