People who received Pfizer-BioNTech or Moderna's mRNA vaccine and later contracted COVID-19, known as breakthrough infections, had milder and shorter illness compared to those who were unvaccinated and infected, according to CDC findings published June 7.
The research, part of the agency's Heroes-Recover study of healthcare workers, involved nearly 4,000 participants who underwent COVID-19 testing for 17 consecutive weeks between December 2020 and April 2021.
To evaluate whether an infection was milder, vaccinated people who developed breakthrough infections were grouped together and compared to unvaccinated, infected participants. Findings showed fully or partially vaccinated people spent fewer total days sick and were 60 percent less likely to develop symptoms than those who weren't vaccinated.
Overall, mRNA vaccines cut the risk of infection by 91 percent among those fully vaccinated and 81 percent for those who were partially vaccinated, the CDC said.
Several findings also indicate vaccinated people are less likely to spread the virus: Those who were vaccinated had 40 percent less detectable virus in their nose and shed the virus for six fewer days 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," the CDC said.
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