Is a booster dose of COVID-19 vaccines effective on newly dominant omicron subvariants among university students? Comparison between BA.1 and BA.2 dominancy

Background: Although the COVID-19 Omicron BA.1 subvariant was initially predominant, the BA.2 subvar-iant has now replaced it. Effectiveness of a booster dose vaccination for BA.2 remains unclear among univer-sity students. Methods: We enrolled 562 Japanese university students who became a close contact and underwent poly-merase chain reaction testing. We compared infection rates and cumulative incidence rates of severe fever among the students according to the COVID-19 vaccine doses received between BA.1-dominant (January 1-March 31, 2022) and BA.2-dominant (April 1-July 31, 2022) periods. Results: Infection rates for BA.1 were 32% with 3 doses, 49% with 2 doses, and 68% in the unvaccinated (P = .008). The odds ratio (OR) for infection following 3 doses during BA.1 was 0.46 (95% confidence interval [CI] = 0.25-0.82, P = .009). Infection rates for BA.2 were 45% with 3 doses, 62% with 2 doses, and 64% in the unvaccinated (P = .02). The OR for infection following 3 doses during BA.2 was 0.50 (95% CI = 0.31-0.82, P = .006). Effectiveness of vaccine for BA.2 tended to decrease for both 3 (45% vs 32%, P = .06) and 2 doses (62% vs 49%, P = .07) compared with those for BA.1. Conclusions: Booster dose effectiveness tended to decrease but remained significant against BA.2 subvariant predominancy among Japanese university students. (c) 2022 Association for Professionals in Infection Control and Epidemiology, Inc. Published by Elsevier Inc. All rights reserved.