Washington, DC–A new CDC study provides strong evidence that mRNA COVID-19 vaccines are highly effective in preventing SARS-CoV-2 infections in real-world conditions among health care personnel, first responders, and other essential workers. These groups are more likely than the general population to be exposed to the virus because of their occupations.
The study looked at the effectiveness of Pfizer-BioNTech and Moderna mRNA vaccines in preventing SARS-CoV-2 infections among 3,950 study participants in six states over a 13-week period from December 14, 2020 to March 13, 2021.
Results showed that following the second dose of vaccine (the recommended number of doses), risk of infection was reduced by 90 percent two or more weeks after vaccination. Following a single dose of either vaccine, the participants’ risk of infection with SARS-CoV-2 was reduced by 80 percent two or more weeks after vaccination.
It takes about two weeks following each dose of vaccine for the body to produce antibodies that protect against infection. As a result, people are considered “partially vaccinated” two weeks after their first dose of mRNA vaccine and “fully vaccinated” two weeks after their second dose. These new vaccine effectiveness findings are consistent with those from Phase 3 clinical trials conducted with the vaccines before they received Emergency Use Authorizations from the Food and Drug Administration. Those clinical trials evaluated vaccine efficacy against COVID-19 disease, while this study evaluated vaccine effectiveness against infection, including infections that did not result in symptoms.
This study is the first of many planned COVID-19 vaccine effectiveness studies CDC is conducting to evaluate the benefits of COVID-19 vaccines in various populations and across different outcomes, such as preventing infections, doctor’s visits, hospitalizations, or deaths. Results from these studies assist the medical and public health experts on the Advisory Committee on Immunization Practices and CDC to make important vaccine policy decisions aimed at saving lives.