Vaccine effectiveness (VE) of three doses of the monovalent (single-strain) mRNA COVID-19 against hospitalization amid Omicron BA.4 and BA.5 subvariant predominance was 68% for up to 4 months but then fell to 36%, estimates a study published yesterday in JAMA Network Open.
The test-negative study, led by researchers from the US Centers for Disease Control and Prevention (CDC) VISION Network, included 82,229 adult emergency department (ED) or urgent care (UC) visits and 21,007 hospitalizations for COVID-19 in 10 states from December 16, 2021, to August 20, 2022.
Among ED and UC patients, median age was 51 years, 60.4% were women, and 23.2% tested positive for COVID-19. Hospitalized patients had a median age of 71 years, 53.4% were women, and 17.1% were diagnosed as having COVID-19.
Booster doses increased protection
Relative to no vaccination, two-dose VE against hospitalization was estimated at 25% at 150 days or more after receipt. Third-dose VE was 68% at 7 to 119 days and 36% thereafter (median days, 235). Fourth-dose VE among patients aged 65 years or older was 66% at 7 to 59 days and 57% thereafter (median days, 88).
Intensive care unit admission or death occurred in 21.4% of hospitalized COVID-19 patients during the BA.1 period and 14.7% during BA.4 and BA.5 (standardized mean difference, 0.17).
"Estimated VE was similar across outcomes, contradicting many past VE studies, including previous studies from the VISION Network, which have tended to show higher vaccine-associated protection for more severe outcomes," the authors wrote. "This could be due to changes in baseline population immunity (eg, most adults now have evidence of prior infection), changes in behavior (eg, decreased use of social distancing and masks during recent months), or residual confounding."
Less severe illness, lower VE
The researchers noted that, across all outcomes, estimated VE was lower than that reported during earlier Delta variant and Omicron BA.1 sublineage predominance.
Intensive care unit admission or death occurred in 21.4% of hospitalized COVID-19 patients during the BA.1 period and 14.7% during BA.4 and BA.5.
"Our findings that COVID-19 mRNA vaccines remain effective and that vaccinated adults weren't as sick during BA.4/BA.5 predominance is encouraging as we look ahead to development of future vaccines and strategies for combatting the virus," coauthor Shaun Grannis, MD, of Regenstrief Institute, said in a Regenstrief news release.
The authorization of additional vaccine booster doses, the waning of vaccine-induced protection, and the emergence of new variants and subvariants, and a growing infection-induced immunity have complicated the estimation of VE, the researchers said.
"As new variants emerge, ongoing monitoring of vaccine effectiveness is critical for informing public health strategies and treatment of patients," coauthor Brian Dixon, PhD, MPA, of Regenstrief Institute, said in the release. "Tracking performance of vaccines will also help develop better vaccines and inform best practices for future pandemics."