A new study from the Centers for Disease Control and Prevention provides strong evidence that mRNA COVID-19 vaccines are highly effective in preventing COVID-19 infections in real-world conditions among health care personnel, first responders and other essential workers.

The study looked at the effectiveness of Pfizer-BioNTech and Moderna mRNA vaccines in preventing infections among 3,950 study participants in six states over a 13-week period from Dec. 14, 2020 to March 13, 2021.

Results showed that following the second dose of vaccine, risk of infection was reduced by 90% two or more weeks after vaccination. Following a single dose of either vaccine, the participants' risk of infection was reduced by 80% two or more weeks after vaccination.

“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,” says CDC Director Rochelle P. Walensky.

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 U.S. 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.