CDC COVID-19 study shows mRNA vaccines reduce risk of infection by 91 percent for fully vaccinated people

A new CDC study finds the mRNA COVID-19 vaccines authorized by the Food and Drug Administration (Pfizer-BioNTech and Moderna) reduce the risk of infection by 91 percent for fully vaccinated people. This adds to the growing body of real-world evidence of their effectiveness. Importantly, this study also is among the first to show that mRNA vaccination benefits people who get COVID-19 despite being fully vaccinated (14 or more days after dose 2) or partially vaccinated (14 or more days after dose 1 to 13 days after dose 2).

"COVID-19 vaccines are a critical tool in overcoming this pandemic," said CDC Director Rochelle P. Walensky, MD, MPH. "Findings from the extended timeframe of this study add to accumulating evidence that mRNA COVID-19 vaccines are effective and should prevent most infections - but that fully vaccinated people who still get COVID-19 are likely to have milder, shorter illness and appear to be less likely to spread the virus to others. These benefits are another important reason to get vaccinated."

The findings come from four weeks of additional data collected in CDC's HEROES-RECOVER study of health care workers, first responders, frontline workers, and other essential workers. These groups are more likely to be exposed to the virus that causes COVID-19 because of their occupations. Preliminary results from this study were first announced in March 2021.

In the new analysis, 3,975 participants completed weekly SARS-CoV-2 testing for 17 consecutive weeks (from December 13, 2020 to April 10, 2021) in eight U.S. locations. Participants self-collected nasal swabs that were laboratory tested for SARS-CoV-2, which is the virus that causes COVID-19. If the tests came back positive, the specimens were further tested to determine the amount of detectable virus in the nose (i.e., viral load) and the number of days that participants tested positive (i.e., viral shedding). Participants were followed over time and the data were analyzed according to vaccination status. To evaluate vaccine benefits, the study investigators accounted for the circulation of SARS-CoV-2 viruses in the area and how consistently participants used personal protective equipment (PPE) at work and in the community. Once fully vaccinated, participants' risk of infection was reduced by 91 percent. After partial vaccination, participants’ risk of infection was reduced by 81 percent. These estimates included symptomatic and asymptomatic infections.

To determine whether COVID-19 illness was milder, study participants who became infected with SARS-CoV-2 were combined into a single group and compared to unvaccinated, infected participants. Several findings indicated that those who became infected after being fully or partially vaccinated were more likely to have a milder and shorter illness compared to those who were unvaccinated. For example, fully or partially vaccinated people who developed COVID-19 spent on average six fewer total days sick and two fewer days sick in bed. They also had about a 60 percent lower risk of developing symptoms, like fever or chills, compared to those who were unvaccinated. Some study participants infected with SARS-CoV-2 did not develop symptoms.

Other study findings suggest that fully or partially vaccinated people who got COVID-19 might be less likely to spread the virus to others. For example, fully or partially vaccinated study participants had 40 percent less detectable virus in their nose (i.e., a lower viral load), and the virus was detected for six fewer days (i.e., viral shedding) compared to those who were unvaccinated when infected. In addition, people who were partially or fully vaccinated were 66 percent less likely to test positive for SARS-CoV-2 infection for more than one week compared to those who were unvaccinated. While these indicators are not a direct measure of a person’s ability to spread the virus, they have been correlated with reduced spread of other viruses, such as varicella and influenza.

Overall, the study findings support CDC's recommendation to get fully vaccinated against COVID-19 as soon as you can. Everyone 12 years and older is now eligible to get a COVID-19 vaccination in the United States. CDC has several surveillance networks that will continue to assess how FDA-authorized COVID-19 vaccines are working in real-world conditions in different settings and in different groups of people, such as different age groups and people with different health statuses.

CDC works 24/7 protecting America’s health, safety and security. Whether disease start at home or abroad, are curable or preventable, chronic or acute, or from human activity or deliberate attack, CDC responds to America’s most pressing health threats. CDC is headquartered in Atlanta and has experts located throughout the United States and the world.