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Authorized bivalent boosters offered additional protection against COVID-19 hospitalization in immunocompetent adults and seniors, a pair of CDC studies showed.

Among adults , a bivalent booster dose showed a vaccine effectiveness (VE) of 57% (95% CI 41-69) against hospitalization and 56% (95% CI 49-62) against emergency department (ED) or urgent care visits versus no vaccination, reported Mark Tenforde, MD, PhD, of the CDC in Atlanta, and co-authors.

And when compared with people who had only received the original mRNA vaccines, VE with the reformulated boosters -- which target the Omicron BA.4/5 subvariants along with the original strain -- was 38-45% against hospitalization and 31-50% against ED/urgent care visits, with higher protection seen for those who had received their last monovalent vaccine dose 11 months or more earlier.

In the of seniors from the IVY Network, the CDC's Diya Surie, MD, and colleagues found a VE of 84% (95% CI 63-92) against hospitalization with the updated boosters versus no vaccination and a VE of 73% (95% CI 57-89) compared with those who had only received doses of the monovalent vaccines.

"Substantial additional protection from a bivalent booster dose was observed when compared with remote monovalent-only mRNA vaccination, which suggests important incremental benefit for persons eligible to receive a bivalent vaccine booster," Surie and researchers noted. "Although prevention of COVID-19 hospitalizations is a core goal of the U.S. vaccination program, bivalent booster dose coverage in the United States remains low among adults aged ≥18 years (16%) and adults aged ≥65 years (36%)."

The protection seen in the two studies -- both of which were published in the Morbidity and Mortality Weekly Report -- was observed during a period of Omicron BA.5 and BQ.1/BQ.1.1 predominance.

VISION Network

For their case-control study, Tenforde's group identified 78,303 ED or urgent care encounters for COVID-like illness from September 13 to November 18 from nine U.S. states in the VISION Network (including 9,009 case patients). Half were ages 18-49 years, 62% were women, 58% were white, 31% had a chronic condition, and 20% had a documented prior COVID infection.

Overall, 31% of these patients were unvaccinated, 24% had received two monovalent doses of mRNA vaccine only, 29% had received three monovalent doses, and 11% had only received four prior doses of the monovalent shots.

A total of 3,905 individuals (5%) in the ED/urgent care cohort received one of the newly authorized bivalent mRNA boosters from Pfizer and Moderna, the vast majority of whom had also received three (43%) or four (51%) prior monovalent doses. Recipients of the bivalent booster tended to be older (median age 68 vs 55 years).

Hospitalization data came from 15,527 hospitalizations for COVID-like illness (including 1,453 case patients) during the same timeframe. The bulk (62%) occurred in those 65 and over, 54% were women, 62% were white, 16% had a prior documented COVID infection, and 95% had a chronic condition. Overall, 26% were unvaccinated, and 22%, 31%, and 16%, respectively, had received two, three, or four doses of the monovalent vaccines only.

A total of 783 hospitalized patients (5%) had received a bivalent booster, with 32% having received three monovalent doses beforehand and 62% having received four prior doses.

For either outcome, individuals were included if they had received a SARS-CoV-2 test in the 2 weeks before or 72 hours before their ED/urgent care visit or hospitalization.

Along with prior suggesting protection against symptomatic infections with the bivalent vaccines, "results from this study also demonstrate protection against ED/[urgent care] encounters and hospitalization during a period when BA.5 and other Omicron sublineage viruses predominated in the United States," noted Tenforde and colleagues.

"These findings support efforts to improve coverage with bivalent vaccines, although optimal timing for receipt of bivalent vaccine booster doses needs to be established," the authors wrote.

Limitations included not accounting for whether or not individuals had a previous SARS-CoV-2 infection in VE estimates, which would give these individuals a level of "background immunity," and for the use of treatments such as nirmatrelvir-ritonavir (Paxlovid), among others.

IVY Network

Surie's group examined data from 22 hospitals in 18 states in the IVY Network, ultimately including 798 patients age 65 and up with COVID-like illness (including 381 controls) who were hospitalized from September 8 to November 30. Median age was 76 years, most were non-Hispanic white (69%), and 95% had at least one underlying medical condition (74% had two or more).

Hospitalizations were matched with immunization data from the electronic medical record, state-based registries, vaccination centers, or patient self-report. Overall, 18% were unvaccinated, 72% had received at least two monovalent doses of vaccine, and 10% had received the bivalent booster.

VE with the booster dose against hospitalization was similar when compared with unvaccinated individuals (84%) and with those who had received their last monovalent dose 12 months earlier (83%).

The findings "are among the first to document real-world evidence that receipt of a bivalent booster dose after completion of at least a primary COVID-19 mRNA vaccination series is protective against COVID-19 hospitalization," wrote Surie and co-authors.

"All eligible persons, especially adults aged ≥65 years, should receive a bivalent booster dose to maximize protection against COVID-19 hospitalization this winter season," the group concluded. "Additional strategies to prevent respiratory illness, such as masking in indoor public spaces, should also be considered, especially in areas where COVID-19 community levels are high."

The study authors noted that the sample size was not large enough to compare VE with the bivalent booster versus the specific number of prior monovalent vaccine doses.

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