In Australians who had never previously had COVID-19, a three-dose COVID-19 vaccination series was 42.0% effective against infection and 81.7% effective against hospitalization or death during Omicron, a new study in Emerging Infectious Diseases finds.
The study was conducted in Western Australia, which had low case counts of COVID-19 until the Omicron variant began to spread beginning in January 2022. The region had strict travel restrictions and local outbreak measures that had significantly reduced the number of COVID-19 cases in the community, leaving a population with low levels of natural immunity, but high (more than 90%) vaccination rates.
To conduct the study, the authors compared the vaccination history of 188,950 positive case-patients and 188,950 negative matched controls from specimens collected from February 1, 2022, to May 31, 2022. Of the 377,900 study participants, 30,420 (8%) were unvaccinated at the time of testing, 84,237 (22%) had received two doses, and 263,243 (70%) had received three doses, the authors said.
Vaccine effectiveness (VE) for preventing lab-confirmed infection was 24.9% (95% confidence interval [CI], 21.2% to 28.4%), increasing to 42.0% (95% CI, 40.2% to 43.6%) for people who had received three doses of vaccine. VE was highest (70.7%) among those who had gotten a booster no more than 30 days before testing.
Ancestral strain vaccines still provide substantial protection against severe disease caused by newer variants.
Severe cases of COVID-19, defined by hospitalizations, and deaths were rare in the study. VE against severe disease for two doses of vaccine was 41.9% (95% CI, 4.8% to 64.5%) and increased to 81.7% (95% CI, 73.9% to 87.2%) for three doses.
The authors concluded, "Our study definitively demonstrates that ancestral strain vaccines still provide substantial protection against severe disease caused by newer variants among a population free from potential confounding of previous immunity conferred by natural infection with an earlier variant."