A monovalent mRNA booster dose offered “limited” protection against COVID-related hospitalizations from the currently circulating Omicron variants, a CDC analysis found, likely due to the waning effects of the vaccines and potentially more immune evasion with BA.4/BA.5.

In immunocompetent adults, vaccine effectiveness (VE) against hospitalization with three doses of mRNA vaccine dropped from 69% (95% CI 62-74) during the BA.1/BA.2 period to 31% (95% CI 7-49) during BA.4/BA.5, reported Diya Surie, MD, of CDC’s COVID-19 Emergency Response Team, and colleagues.

During the period of BA.4/BA.5 specifically, protection against hospitalization declined from 60% (95% CI 12-81) in the first 4 months from the third dose to 29% (95% CI 3-48) thereafter, according to their findings in the Morbidity and Mortality Weekly Report (MMWR).

A second study, also published in MMWR, showed similar declines for individuals with immunocompromising conditions during the period of Omicron predominance. VE against COVID-related hospitalizations with three doses was 67% (95% CI 63-71) during the BA.1 period in this population, which fell to 32% (95% CI 22-42) during periods of BA.2/BA.2.12.1 and BA.4/BA.5 circulation, according to researchers led by Amadea Britton, MD, also of the CDC’s COVID-19 Emergency Response Team.

A fourth dose appeared to restore some of the protection in these immunocompromised individuals during the periods of BA.2/BA.2.12.1 and BA.4/BA.5 circulation, with VE landing at 43% (95% CI 27-56) with the additional dose.

Researchers from both studies argued that the findings demonstrate the importance of staying up to date with COVID-19 vaccinations and suggested that the newer bivalent mRNA shots may offer superior protection, given that they target circulating BA.4/BA.5 sublineages, as well as the ancestral strain of SARS-CoV-2.

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