Study on COVID vaccination and contagiousness misrepresented
CLAIM: A recent study in the New England Journal of Medicine found that unvaccinated COVID-19 patients remain contagious for less time than those who were vaccinated or had received booster shots.
AP’S ASSESSMENT: False. The study found no significant overall difference in how long patients with the delta or omicron variants were capable of spreading the virus, regardless of vaccination status, according to two authors of the study.
THE FACTS: Research teams from a trio of Massachusetts medical institutions conducted the study of 66 participants from July 2021 to January 2022. Shortly after its publication on July 21, conservative news website The National Pulse ran an article falsely suggesting the study found vaccinated patients remain contagious longer over the course of their infection.
In the study, a higher percentage of unvaccinated participants tested negative on PCR tests 10 days and 15 days out from their first positive test, compared to vaccinated and boosted participants. The article concluded that these data points alone meant “individuals who did not receive a COVID-19 vaccine were contagious for a shorter period of time.”
But that conclusion is incorrect, according to Drs. Amy Barczak and Mark Siedner, researchers at Massachusetts General Hospital who worked on the study.
The information in the article is skewed because it used only select information from the study, examining only data around PCR tests and two time periods. But the study relied on two forms of COVID-19 test, with each participant tested repeatedly over the course of their illness. It found that while some individual participants tested negative sooner than others, on the whole, unvaccinated, vaccinated and boosted patients all tested positive for roughly the same period of time.
“There’s no difference in any group on either side of the study,” Siedner told The Associated Press. He added, “Any single data point on any single day is not taking into account the overall design of the study.”
The study participants were tested using the now-ubiquitous PCR tests as well as a more complex process known as a viral culture, in which scientists attempt to grow the virus from a sample taken from a patient’s nose. That additional form of testing — which requires the use of specialized high-security labs — is better at determining whether a person can still spread the virus, Siedner said.
“We learned very early in the epidemic to stop testing people by PCR after they’re positive once, because many people are positive for weeks or months,” he said. “But a viral culture allows us to say, ‘OK, is there still something that can replicate there — and therefore, is there still something that potentially could put someone at risk forgetting giving them a virus to someone else?’”
The rate at which study participants began testing negative for the virus was about the same, regardless of their vaccination status, the coronavirus variant they’d contracted or the testing method used, according to the researchers.
Reached via Twitter on Tuesday, a National Pulse spokesperson told the AP, “We report the News. We link to sources. People can therefore read those for themselves.”
The study concluded that many infected people, including those who have been vaccinated and boosted, may continue to be contagious beyond the Centers for Disease Control and Prevention’s recommended five-day isolation period.
“Vaccination is clearly protective against getting infected, at least to some extent. And it’s very protective against getting very sick once you’re infected,” Barczak said. “But it may be that once the horse is out of the barn, and the infection is established, vaccination may not be super helpful in helping you clear that virus more quickly.”
This is part of AP’s effort to address widely shared misinformation, including work with outside companies and organizations to add factual context to misleading content that is circulating online. Learn more about fact-checking at AP.