Can vaccinated people still spread COVID-19? Huge study tackles question
A federally funded study taking place at 21 college campuses will test how well Moderna's COVID-19 shot prevents vaccinated people from spreading the coronavirus, The Washington Post reported.
Clinical trials have shown that the Moderna vaccine is more than 94% efficacious at preventing illness from COVID-19, and that the shots are especially protective against severe disease, hospitalization and death from the virus. However, the clinical trials were not designed to answer an important question: Can vaccinated people still carry the coronavirus in their nose and mouth and unwittingly spread it to others?
Real-world studies in Israel and the U.K. hint that COVID-19 vaccines cut down the risk of both symptomatic and asymptomatic infections, meaning those without any outward signs of illness, the Post reported. These two studies each focused on the Pfizer and AstraZeneca vaccines, respectively. Another study in 4,000 health care and essential workers in the U.S. provided additional evidence that the Pfizer and Moderna vaccines protect against all infections, including those without symptoms, Stat News reported.
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While these studies provide clues that vaccinated people may be less likely to spread the virus, because they appear to avoid infection overall, they cannot confirm this conclusively. The new college campus study, called PreventCOVIDU, will attempt to directly answer the question through contact tracing — where COVID-19 infections are tracked among vaccinated people, unvaccinated people and a large group of their close contacts.
Tracking if and how infections ripple through this large group of people should help reveal how often vaccinated people pass the virus to those around them, regardless of whether the vaccinated person falls ill.
"This study is addressing the important issue about what does it mean to be vaccinated, as far as your risk for transmitting SARS-CoV-2 to people in your bubble of trust," Dr. Lilly Immergluck, a pediatric infectious-disease specialist at Morehouse School of Medicine in Atlanta, told the Post.
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PreventCOVIDU will include 12,000 university students ages 18 to 26, according to the study's recruitment website. The students will be randomly split into two groups; half will receive their first Moderna vaccine dose right away, while the other half will be vaccinated four months later. The entire trial will take place over a five-month period.
All participants will take daily nose swabs throughout the trial, so the study organizers can track when COVID-19 infections occur and in whom. The swabs will also help them calculate the quantity of viral particles in each infected person's nose and the genetic sequence of the virus they contracted. These data points will help determine whether viral load — the amount of virus in a person's system — is linked to the risk of transmission. They will also show whether the Moderna vaccine provides different levels of protection against different strains of the virus and whether vaccinated people are more likely to spread certain strains over others.
A phone app will remind participants to swab their noses daily. They will also complete daily questionnaires about their symptoms, provide blood samples at several points in the study and undergo routine COVID-19 screenings through their university testing systems.
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The main study participants will also identify a group of their close contacts, meaning people who could be at risk of catching COVID-19 if the college students test positive for the virus. In total, the trial organizers hope to collect data from 25,500 close contacts. (Both the main participants and their close contacts will be compensated for their participation in the trial.)
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Roommates and coworkers of the main participants will be considered "prospective close contacts (PCC)," and in the event a participant tests positive for the virus, they'll also identify "case-ascertained close contacts (CACC)," or additional people they may have exposed to the virus in recent days.
Following a confirmed infection in a participant, close contacts who participate in the study will answer weekly symptom questionnaires, take daily swab tests for two weeks after the positive test and may potentially provide two blood samples. PPC participants will undergo routine COVID-19 screening at their university throughout the trial, while CACC's will do so for just one month following their potential exposure.
Results from the study are expected "later this year," according to the study website.
You can read more about the trial's first participants at The Washington Post.
Originally published on Live Science.
Nicoletta Lanese is the health channel editor at Live Science and was previously a news editor and staff writer at the site. She is a recipient of the 2026 AHCJ International Health Study Fellowship, with a project focused on antibiotic stewardship practices in Japan and the U.S. They hold a graduate certificate in science communication from UC Santa Cruz and degrees in neuroscience and dance from the University of Florida. Beyond Live Science, Lanese's work has appeared in The Scientist, Science News, the Mercury News, Mongabay and Stanford Medicine Magazine, among other outlets. Based in NYC, she also remains involved in dance and performs in local choreographers' work.