Updated COVID boosters add to benefit of single-strain doses

Vial of bivalent COVID booster vaccine
Vial of bivalent COVID booster vaccine

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A US study of the vaccine effectiveness (VE) of the new bivalent mRNA COVID-19 boosters estimates that they confer 28% to 56% more protection against symptomatic infections than two to four doses of the original mRNA vaccines.

Researchers from the Centers for Disease Control and Prevention (CDC) and Eagle Health Analytics in San Antonio published the research yesterday in Morbidity and Mortality Weekly Report.

Designed to address waning monovalent VE

On Sep 1, the CDC's Advisory Committee on Immunization Practices (ACIP) recommended the bivalent (two-strain) vaccines, which include components of both wild-type SARS-CoV-2 and the Omicron BA.4/BA.5 strains, for Americans 12 years and older (Pfizer/BioNTech vaccine) or adults only (Moderna vaccine) who had completed a primary monovalent (single-strain) vaccine series at least 2 months earlier.

On Oct 12, the ACIP recommendation was expanded to include children aged 5 to 11 years. The new vaccines were developed to address the waning effectiveness of monovalent vaccines against Omicron, but no clinical efficacy data on the bivalent formulations were available at recommendation.

The CDC researchers assessed bivalent booster VE using the national Increasing Community Access to Testing program, which included data from 360,626 COVID-19 nucleic acid amplification tests (NAATs) performed from Sep 14 to Nov 11 at 9,995 retail pharmacies in adults who had COVID-19 symptoms but no immunocompromising conditions. Most tests (81%) were conducted during Omicron BA.4/BA.5 predominance.

Of the 360,626 NAATs, 34% were positive for COVID-19. Among the case-patients, 24% were unvaccinated, 72% had received two to four monovalent vaccine doses but no bivalent booster dose, and 5% had received a bivalent dose. Among the 238,939 uninfected controls, 30% were unvaccinated, 63% had received two to four monovalent vaccine doses but no bivalent booster dose, and 7% had gotten a bivalent booster dose.

The median interval between bivalent booster vaccination and COVID-19 testing was 1 month and did not vary by test result. A greater proportion of participants who tested negative at least 90 days before the current test (43%) reported a COVID-19 infection than those who tested positive (22%) in that timeframe.

Greater relative VE over time

Absolute VE (aVE) of a bivalent booster dose after two or more monovalent doses was 28% among participants aged 50 to 64 and 22% among those 65 years and older, although it varied slightly by number of previous monovalent doses. Of adults 18 to 49 years old who received at least two monovalent doses, aVE was 43% higher than those of older age-groups and didn't vary by number of previous monovalent doses.

VE of a bivalent booster dose relative to that of two or more monovalent doses 2 to 3 or 8 months earlier was 30% and 56%, respectively, among 18- to 49-year-olds, 31% and 48% among those aged 50 to 64, and 28% and 43% among those 65 years and older.

The study authors said the results were the first published VE estimates for the bivalent booster vaccines. They said the aVE should be interpreted carefully because unvaccinated people might behave differently or be at different levels of risk for infection than vaccinated people, important factors considering that more than 90% of American adults have received at least one COVID-19 vaccine dose.

"Bivalent mRNA booster doses provide additional protection against symptomatic SARS-CoV-2 in immunocompetent persons who previously received monovalent vaccine only, with relative benefits increasing with time since receipt of the most recent monovalent vaccine dose," the researchers wrote. "Staying up to date with COVID-19 vaccination, including getting a bivalent booster dose when eligible, is critical to maximizing protection against COVID-19."

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