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mRNA COVID-19 Vaccine Efficacy Against Infection or Death in an At-Risk Study Group

A study of mRNA vaccination in older people and people with comorbidities found vaccine efficacy of 69% against COVID-19 infection and 86% against death.

One study, published this week in Annals of Internal Medicine, sought to determine the efficacy of messenger RNA COVID-19 vaccines in racially and ethnically diverse, elderly Americans.

The study authors wrote, “The real-world effectiveness of the 2 mRNA vaccines in ethnically and racially diverse populations across the entire United States is not well characterized, especially in more vulnerable populations, such as elderly persons with high comorbidity burden.”

The investigators used the US Department of Veteran Affairs health care system data for this study, due to its inclusion of a large population of older adults with comorbidities. They used a target emulation study design to estimate the effectiveness of the Pfizer-BioNTech and Moderna mRNA vaccines against COVID-19 infection and death.

The study compared newly vaccinated people with a matched unvaccinated control cohort. The investigators included 2103790 persons who received at least 1 mRNA vaccine dose from December 11, 2020-March 25, 2021; 2099871 (55.5%) received Moderna and 933852 (45.5%) received Pfizer-BioNTech.

Both the vaccinated and unvaccinated groups were predominantly male (92.9% versus 93.4% male, respectively), and both had an average participant age of 68.7 years. The vaccinated group was 17.3% Black and 6.5% Hispanic, and the unvaccinated group was 17.0% Black and 6.1% Hispanic; this study’s cohorts had substantially more racial and ethnic diversity than most prior studies.

At 7 or more days after initial mRNA vaccination, efficacy was 69% against COVID-19 infection and 86% effective against COVID-19-related death. This efficacy remained similar 3 months after the original follow-up date, for an average follow-up of 101 days. Vaccine efficacy, predictably, decreased as age and comorbidities increased.

The investigators found COVID-19 vaccine efficacy against infection was significantly lower than previous research had suggested, but vaccine efficacy against death was high. The lower observed vaccine efficacy than reported in previous phase 3 clinical trials is likely due to this study’s inclusion of older people and people with comorbidities.

The investigators concluded by noting that additional mitigation measures, such as face masks and social distancing, are vital for controlling COVID-19 infection.