COVID-19 Vaccine Reduces Severity, Length, Viral Load for Those Who Still Get Infected


People who contract COVID-19 even after vaccination are likely to have a lower viral load, experience a shorter infection time and have milder symptoms than those who are unvaccinated, according to research that includes data from ongoing University of Arizona Health Sciences studies.

“If you get vaccinated, about 90% of the time you’re not going to get COVID-19,” said Dr. Jeff Burgess, 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 study, otherwise know as AZ HEROES. “Even if you do get it, there will be less of the virus in you and your illness is likely to be much milder.”

While the COVID-19 vaccines are proving to be highly effective in preventing COVID-19 infection, no vaccine is 100% effective, and breakthrough infections do occur. Among 3,975 participants in two studies, SARS-CoV-2 infections were identified in five participants who were fully vaccinated and 11 who were partially vaccinated, as well as in 156 unvaccinated participants. Approximately half of the participants were from Arizona study sites.

Researchers found that study participants who were partially or fully vaccinated with the Pfizer and Moderna messenger RNA vaccines at the time of infection had a viral load that was 40% less than that of unvaccinated participants. Viral load – the amount of SARS-CoV-2 virus found in a test sample – is not an indicator of how contagious an individual is, though early COVID-19 research suggests viral load could play a role in disease severity and secondary transmission.

In addition to disease severity, researchers looked at infection longevity. The majority of infections among unvaccinated participants were detected for two or more weeks, compared with only one week among vaccinated participants. That represents a 66% reduction in the risk that a vaccinated person will have a confirmed infection for more than one week.

Additionally, the risk of having COVID-19 with an accompanying fever was 58% lower for vaccinated participants, who reported two fewer days sick in bed, on average, and an overall length of illness that was six days shorter than that of unvaccinated people.