Effectiveness of CoronaVac primary series with and without booster against hospitalized COVID-19 during the Omicron-predominant epidemic wave in the Philippines: a test-negative case-control study.

Background: CoronaVac (Sinovac) was initially effective against symptomatic COVID-19 and severe outcomes, but its performance against the immune-evasive Omicron variant remains uncertain. This study evaluated the effectiveness of CoronaVac against hospitalized COVID-19 among Filipino adults during the Omicron-dominant wave.

Research design and methods: We conducted a test-negative case-control study from November 2022 to November 2023 in three tertiary government hospitals in the Philippines. Adults hospitalized with acute respiratory illness (ARI) were enrolled. Cases tested positive for SARS-CoV-2 by RT-PCR; controls tested negative. Vaccination status was confirmed via vaccination cards or the national registry. Conditional logistic regression estimated vaccine effectiveness (VE). Genomic sequencing identified circulating variants.

Results: Among 2,365 participants, 165 (7.0%) were COVID-19 positive. In an age-matched analysis (104 cases, 408 controls), two CoronaVac doses provided 61.3% protection (95% CI: 5.3-84.2%) against critical illness. A heterologous booster conferred 65.9% protection against severe disease, 90.1% against critical illness, and 60.5% against death. Sequencing of 23/46 samples confirmed Omicron XBB-like variants.

Conclusion: Two doses of CoronaVac offered moderate protection against severe COVID-19. Heterologous boosters significantly improved protection, especially against critical illness and death, supporting ongoing booster campaigns after inactivated vaccine priming.