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 subvariant has now replaced it. Effectiveness of a booster dose vaccination for BA.2 remains unclear among university students. METHODS: We enrolled 562 Japanese university students who became a close contact and underwent polymerase 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.