COVID-19 Resource Center

COVID-19 Resource Center

Disclosures: Thompson and Walensky report no relevant financial disclosures.
March 29, 2021
2 min read

Real-world data show mRNA vaccines reduce infection risk by 90%

Disclosures: Thompson and Walensky report no relevant financial disclosures.
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Real-world data reported Monday in MMWR showed that the two COVID-19 messenger RNA vaccines authorized for use in the United States reduce the risk for SARS-CoV-2 infection by 90% 2 or more weeks after administration of the second dose.

A study of nearly 4,000 health care personnel, first responders and other frontline and essential workers also showed the messenger RNA (mRNA) vaccines manufactured by Pfizer-BioNTech and Moderna are 80% effective after one dose.

CDC mRNA graphic
Source: CDC

“Our national vaccination efforts are working,” CDC Director Rochelle P. Walensky, MD, MPH, said in a White House press briefing.

In an earlier statement, Walensky said the data “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.”

“These findings also underscore the importance of getting both of the recommended doses of the vaccine in order to get the greatest level of protection against COVID-19, especially, as our concerns about variants escalate,” she said later at the briefing.

Rochelle P. Walensky

For the study, Mark G. Thompson, PhD, deputy chief for science in the Epidemiology and Prevention Branch of the CDC’s Influenza Division, and colleagues assessed the vaccines’ efficacy among 3,950 health care workers, first responders and frontline workers with no documented SARS-CoV-2 infection in six states between Dec. 14, 2020, and March 13, 2021. Of the participants, 2,479 (62.8%) had received both doses of an mRNA vaccine and 477 (12.1%) received a single dose.

According to Thompson and colleagues, there were 0.04 infections reported per 1,000 person-days among fully immunized participants — those with at least 2 weeks since their second dose — and 0.19 infections reported per 1,000 person days among participants who had received their first dose at least 14 days ago. By comparison, there were 1.38 reported infections per 1,000 person days among unvaccinated participants.

“Given that there is uncertainty related to the number of days required to develop immunity postvaccination, future research examining vaccine effectiveness at different intervals is warranted,” the authors wrote. “These interim vaccine effectiveness findings for both Pfizer-BioNTech’s and Moderna’s mRNA vaccines in real-world conditions complement and expand upon the vaccine effectiveness estimates from other recent studies and demonstrate that current vaccination efforts are resulting in substantial preventive benefits among working-age adults.”


Press Release.

Thompson MG, et al. MMWR Morbid Mortal Wkly Rep. 2021;doi:10.15585/mmwr.mm7013e3.