The study was conducted in partnership with the NBA.
It examined nearly 20,000 viral samples from the league’s occupational health program.
It found vaccinated people cleared their infections an average of two days faster.
A new study of NBA players, staff, and household members provides yet more evidence that fully vaccinated people who get a breakthrough case of COVID-19 may be less infectious than their unvaccinated peers.
Last July, the US Centers for Disease Control and Prevention walked back earlier guidance that had said vaccinated people need not wear masks indoors, saying they should do so in areas of the country experiencing high rates of infection. At the time, CDC Director Rochelle Walensky cited the rise of the highly transmissible Delta variant and “worrisome” new science that indicated vaccinated individuals “may be contagious and spread the virus to others.”
The new study, published by the peer-reviewed New England Journal of Medicine, does not challenge that claim — researchers found that people with breakthrough infections may be just as contagious as unvaccinated people.
However, researchers found that individuals with breakthrough illnesses are infectious for a shorter period of time, reducing the opportunity for them to pass the virus onto others. What’s more, previous evidence suggests vaccinated people are also far less likely to be infected in the first place, especially if they have received a booster shot.
The vaccinated have a shorter transmission window, the study found
To reach their findings, scientists partnered with the National Basketball Association and analyzed 19,941 viral samples from 173 people enrolled in the professional league’s occupational health program between November 2020 and August 2021. Under the program, NBA players were subject to regular testing for COVID-19.
According to the study, vaccinated people remained infected for 5.5 days, on average, compared to 7.5 days for unvaccinated people. That’s 48 hours — or roughly 36% — less time to infect others.
Researchers found no difference between those who received the two-dose Pfizer-BioNTech mRNA vaccine and the one-dose viral vector vaccine from Johnson & Johnson. (Moderna’s mRNA vaccine was not assessed due to a small sample size.)
“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,” study co-author Stephen Kissler said in a statement. Kissler is a research fellow at the Department of Immunology and Infectious Diseases at the Harvard T.H. Chan School of Public Health, which led the study.
The findings support earlier claims about vaccines
While the NBA partnership provided a steady stream of samples, it also produced a study population that was overwhelmingly young, healthy, and male, and therefore not representative of the public at large.
It was also conducted before the rise of Omicron, the latest variant of concern. While it’s not yet clear whether Omicron is more infectious or virulent than Delta, more research is needed to see how well our vaccines fend it off.
Nevertheless, the findings support what Dr. Anthony Fauci, the US’s top infectious disease expert, said last summer: that vaccinated people are “less likely” to spread the virus. Previous studies have also found that vaccinated people typically experience shorter, milder symptoms if they are infected, as well as a faster pace of clearing the virus, all of which points to a narrower window for transmission.
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