Two studies show effectiveness of mRNA boosters against omicron variant

Three doses of mRNA vaccine were as effective for preventing hospitalization from the omicron variant as two doses were against previous COVID-19 variants, according to a study published by The BMJ on March 9. It used data on 11,690 adults admitted to 21 U.S. hospitals in 2021 and 2022: 5,728 with COVID-19 (cases) and 5,962 without (controls). Effectiveness of two vaccine doses was 85% against the alpha and delta variants, but only 65% against the omicron variant; three doses brought that up to 86%. In-hospital mortality was 7.6% for alpha, 12.2% for delta, and 7.1% for omicron. Illness severity was lower for vaccinated patients than for unvaccinated patients for all three variants. Limitations of the study include that it only evaluated inpatients “and thus does not inform vaccine effectiveness against mild covid-19 or differences in disease severity among people with SARS-CoV-2 variants in the outpatient setting,” the authors noted. However, they concluded that the results “suggest that vaccination against covid-19, including a third dose of an mRNA vaccine, is critical for protecting populations against covid-19-associated morbidity and mortality.”

A study from Qatar found similar effectiveness of boosters against omicron and compared results between the two mRNA vaccines, with results published by the New England Journal of Medicine on March 9. It matched 281,093 people who got a Pfizer-BioNTech booster and 92,829 who got a Moderna booster with patients who weren't boosted but received their second shots at the same time as the boosted patients. In the Pfizer-BioNTech vaccine cohort, the cumulative incidence of symptomatic omicron infection was 2.4% with a booster versus 4.5% without. The booster's effectiveness against hospitalization or death due to omicron, as compared with the primary series, was 76.5%. In the Moderna cohort, the cumulative incidence of symptomatic omicron infection was 1.0% in boosted patients versus 1.9% in those not boosted. Too few severe cases were found in the Moderna-vaccinated cohorts to estimate the booster's effect on this outcome, one of the study's limitations noted by the authors. They concluded that their findings “may suggest the need for the development of a new generation of vaccines that target a broad range of variants to confront virus evolution, rather than a continued strategy of repeated boosters with existing vaccines.”

In other recent COVID-19 research, a small study published by Clinical Infectious Diseases on March 10 highlighted the transmissibility of the omicron variant BA.2, known as the stealth variant. It looked at an outbreak in Hong Kong that included 768 patients, 432 of them residents, visitors, or staff of a single housing estate. The outbreak had a short doubling time of 1.28 days and was controlled by locking down three buildings in the estate. “This outbreak illustrates that BA.2 can be transmitted efficiently in a population where universal masking has been adopted since the beginning of the COVID-19 pandemic,” the study authors wrote.

Finally, the effects of COVID-19 on the brain were described in a study published by Nature on March 7. Participants in an ongoing multi-modal brain imaging study (ages 51 to 81 years) were compared by whether they had COVID-19 (n=401) or did not (n=384). Comparison of changes in their brains over time found that those who had COVID-19 between scans had greater reduction in grey matter thickness and tissue contrast in the orbitofrontal cortex and parahippocampal gyrus, greater changes in markers of tissue damage in regions connected to the primary olfactory cortex, and greater reduction in global brain size, as well as greater cognitive decline on average. The differences remained after exclusion of the 15 patients hospitalized for COVID-19. The results “may be the in vivo hallmarks of a degenerative spread of the disease via olfactory pathways, of neuroinflammatory events, or of the loss of sensory input due to anosmia,” the authors said. More research is needed to determine whether the changes are reversible or will persist long term, they added.