A new study suggests the messenger RNA vaccines produced by Moderna and the Pfizer-BioNTech partnership appeared to be 90% effective in preventing Covid-19 infection in a real-world setting.
The study was released in Morbidity and Mortality Weekly Report, an online journal published by the Centers for Disease Control and Prevention.
The study followed nearly 4,000 health care workers, first responders, and other essential workers in eight U.S. locations as the first Covid vaccines were rolled out starting in December. Participants were tested weekly to look for all cases of Covid infection, even asymptomatic ones.
In the period from Dec. 14, 2020, to March 13, 2021, nearly 75% of the workers in the cohort received at least one dose of one of the mRNA vaccines. Both are given in a two-dose schedule.
There were 161 Covid infections in the unvaccinated workers, compared with 16 in workers who had received only one dose by the time of their infection and only three infections in people who had received both doses and were two weeks out from their second dose. The vaccine effectiveness following two doses was 90% — roughly in line with the 95% and 94% that the Pfizer and Moderna vaccines showed, respectively, in the clinical trials that supported their emergency use authorizations.
The study was not conducted in such a way as to allow the researchers to estimate effectiveness measures for each of the vaccines.
The study suggested that even the first dose of vaccine was 80% effective at preventing infection, starting from two weeks after receipt of that dose. That estimate, however, only applies to the brief period until the second dose was administered. The study was not designed to test how well the vaccine works if an individual does not receive the second dose.
The majority of infections — 58% — detected in the study were found because participants were tested weekly to look for infections; 42% of the infections were identified when study participants were tested after developing symptoms. Most of the people who tested positive in the study had some Covid-related symptoms, though 10.7% had none. Only 23% of the people who became infected needed medical care and only two were hospitalized. There were no Covid deaths in the study.