COVID-19 vaccines reduce viral load, severity in breakthrough cases, studies find

Fully vaccinated individuals who then go on to contract COVID-19 are likely to have milder symptoms, a shorter infection time and a lower viral load, ongoing studies suggest. The data, compiled by University of Arizona Health Sciences researchers, included 3,975 participants across two studies, and saw just five fully vaccinated individuals develop SARS-CoV-2 infections compared to 156 unvaccinated individuals. 

The researchers also noted 11 infections among participants who were partially vaccinated. Participants who had been vaccinated had received either the Pfizer-BioNTech or Moderna mRNA vaccines. Infections detected in fully vaccinated individuals are considered "breakthrough cases." Among those cases, researchers noted a viral load about 40% less than what was detected in unvaccinated individuals who had been infected. They also were only able to detect the virus in infected vaccinated individuals for about a week, whereas unvaccinated individuals remained infected for two or more weeks. 

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"If you get vaccinated, about 90% of the time you’re not going to get COVID-19," Jeff Burgess, MD, MS, MPH, associate dean for research and professor at the Mel and Enid Zuckerman College of Public Health and principal investigator of the Arizona Healthcare, Emergency Response, and Other Essential Workers Surveillance (AZ HEROES) study, said. "Even if you do get it, there will be less of the virus in you and your illness is likely to be much milder."

The ongoing studies are now incorporating data pertaining to variants but had assessed the vaccines’ efficacy against infection from Dec. 14 to April 10. Researchers said they found two doses of an mRNA COVID-19 vaccine were 91% effective against SARS-CoV-2 infection, while a single dose was around 81% effective. They said their findings were on par with previously published data from the Centers for Disease Control and Prevention (CDC). 

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"We are still seeing the same high levels of vaccine effectiveness, so we feel good about that," Burgess said, in a news release published on EurekAlert.org. "But more importantly, we’ve added a number of measures of the severity of infection among individuals who have been vaccinated as a comparison to those who haven’t and we measured how much virus there is and for how long."