Abstract

In Japan, the SARS-CoV-2 Omicron (B.1.1.529) variant has been spreading rapidly since the first case was detected in November 2021. Although this variant has been reported to have immune escape potential and SARS-CoV-2 mRNA vaccines have been reported to be less effective against this variant, the protective effect can be increased by a third dose of vaccine. In National Hospital Organization Hakodate National Hospital, an outbreak of the Omicron variant involving more than 30 Healthcare Workers (HCWs) and 90 hospital inpatients occurred in February 2022, despite most hospital staff having received three doses of vaccine. 95% confidence intervals for the ratio of the median antibody titers were 0.71 to 1.86 and there was no significant difference in antibody titers after the third vaccination between infected and non-infected HCWs. Although all cases of SARS-CoV-2 infection in HCWs were asymptomatic or mild, a third dose of mRNA vaccine may be insufficient to prevent infection with the Omicron variant or vaccine-induced antibody titers may not reflect the preventive effect against Omicron.

Keywords: SARS-CoV-2 Omicron variant; Outbreak; mRNA vaccine; Booster dose; Healthcare workers