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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).
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.
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.
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
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.