Harvard study: Vaccinated people with COVID-19 may be infectious for a shorter period of time

The study found that breakthrough cases are infectious for two days less than cases in unvaccinated individuals.

A nurse administers a COVID test provided by Harvard Street Neighborhood Health Center at Prince Hall Grand Lodge in Boston. Jessica Rinaldi / The Boston Globe

Getting the COVID-19 vaccine may not only offer protection against severe complications from the disease. It may also decrease the odds that individuals who get the virus spread it to others — and we’re getting a better idea why.

According to a new study led by Harvard T.H. Chan School of Public Health and published Wednesday in the New England Journal of Medicine, vaccinated individuals who contract COVID-19 may be less likely to spread the virus because they “shed” it for a shorter period of time than unvaccinated people who are infected.

Researchers found that breakthrough infections in vaccinated individuals cleared in an average of 5.5 days, roughly two days quicker than infections in unvaccinated individuals, who remained contagious for an average of 7.5 days.


While it’s possible that people with breakthrough cases are still as infectious as unvaccinated people during the early stages of their infection, the study authors wrote that their shorter infectiousness period means they’re less likely to spread the virus over time.

The findings backs up the consensus among health experts that vaccinated individuals are less likely to spread COVID-19, though the exact reason had been somewhat muddled.

Following the emergence of the more contagious delta variant this summer, some health officials, including the Centers for Disease Control and Prevention, suggested that the reason fully vaccinated individuals could transmit the new strain easier than previous variants was because it resulted in a higher peak viral load — or higher level of virus particles — in the infected individual.

However, the new Harvard study suggests that notion isn’t correct.

In the study, researchers said they found no meaningful difference in peak viral loads from the different COVID-19 variants, suggesting the delta strain’s increased transmissibility was not in fact due to higher virus production.

“Our work provides the most detailed information to date about how viral concentrations change in the body across the full duration of SARS-CoV-2 infection,” Stephen Kissler, postdoctoral immunology fellow at Harvard and co-author of the study, said in a statement.


The study was conducted in partnership with the NBA’s occupational health program, collecting nearly 20,000 coronavirus samples between Nov. 28, 2020 and Aug. 11, 2021 from 173 players and league employees. The samples included a number of different variants, including the original alpha and more recent delta strains, detected in both vaccinated and unvaccinated individuals.

Still, the study’s authors cautioned that further research is necessary, since the study predominantly involved young, male, and healthy participants, and is therefore not representative of the general population.

Currently, the CDC recommends that those who test positive for COVID-19 self-isolate for 10 days, regardless of whether they are vaccinated or not.

The findings come amid an increase in breakthrough infections, as officials urge all adults to get an additional booster shot to increase their immunity.

Over the last few weeks, breakthrough cases have accounted for over a third of all new COVID-19 infections in Massachusetts. Unvaccinated individuals still account for a disproportionate number of cases in the state, in which over 70 percent of the population is fully vaccinated. As of last week, just 0.05 percent of fully vaccinated residents had been hospitalized due to COVID-19.


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