Real-World Study Shows Power of Pfizer, Moderna Vaccines to Prevent COVID-19

A real-world study shows that even when people who get the Pfizer and Moderna COVID-19 vaccines do have "breakthrough" infections, those illnesses are mild.

The study, from researchers at the U.S. Centers for Disease Control and Prevention, is among the first to show that people who get COVID-19 despite being fully or partially vaccinated are less likely to have serious illness or to infect others.

The new findings were culled from four weeks of additional data gathered in a CDC study of U.S. health care workers, first responders, frontline workers, and other essential workers. All of these people have a higher risk for exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The analysis included nearly 4,000 participants who completed weekly SARS-CoV-2 testing for 17 weeks (Dec. 13, 2020, to April 10, 2021) in eight locations across the United States.

The researchers found that the risk for symptomatic and asymptomatic infection was reduced by 81 percent after partial vaccination and by 91 percent after full vaccination. Furthermore, among people who became infected, those who were partially or fully vaccinated were more likely to have a milder and shorter illness than those who were not vaccinated.

The investigators also found that vaccinated people who develop COVID-19 might be less likely to infect others. Fully or partially vaccinated people had 40 percent less detectable virus in their nose, and the virus was detected for six fewer days compared with those who were unvaccinated when infected. 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 with those who were unvaccinated.