March 29, 2021
The COVID-19 vaccines developed by Pfizer and Moderna are highly effective in real-world settings — not just in clinical trials, according to a new U.S. Centers for Disease Control and Prevention study.
The two-dose vaccines reduced infection risk by 90% in a study of 3,950 health care personnel, first responders and other essential employees. A single does of either vaccine reduced infection risk by 80% two or more weeks after the shot.
Researchers focused the study on front-line workers because they have a higher infection risk compared to the general population. They also were among the first people to become fully vaccinated.
The study confirmed the findings of the clinical trials conducted before the vaccines received emergency use authorizations from the U.S. Food and Drug Administration in December.
During clinical trials, the Pfizer vaccine had a 95% efficacy rating; Moderna's vaccine had a 94.1% efficacy rating. But those ratings detailed the vaccines' abilities simply to prevent symptomatic COVID-19. The CDC study measured their abilities to prevent all coronavirus infections, including asymptomatic cases.
"This study shows that our national vaccination efforts are working," CDC Director Rochelle P. Walensky said. "These findings should offer hope to the millions of Americans receiving COVID-19 vaccines each day and to those who will have the opportunity to roll up their sleeves and get vaccinated in the weeks ahead. The authorized vaccines are the key tool that will help bring an end to this devastating pandemic."
To measure the effectiveness of the vaccines, the study participants were followed for 13 weeks between Dec. 14 and March 13. Participants were asked to send in self-collected nasal swabs each week for laboratory testing, regardless of whether they had developed symptoms of illness.
Only 10.7% of the infections in the study were asymptomatic. But more than half of the infections that occurred during the study period were identified by testing before people developed symptoms or knew they were infected.
The CDC plans to conduct more vaccine studies in various populations and focused on different outcomes including doctor's visits, hospitalizations and deaths.
The single-dose Johnson & Johnson COVID-19 vaccine, which was authorized in February, was not included in the study. Clinical trials have shown the vaccine to be 66% effective at preventing moderate to severe COVID-19 and 85% effective at preventing severe illnesses that can lead to hospitalization and death.
The Johnson & Johnson vaccine uses a genetically modified adenovirus, a virus, that causes the common cold, to trigger an immune response. The Pfizer and Moderna vaccines use synthetic messenger RNA to build immunity.