With Covid-19 vaccines rolling out across the United States, the beginning of the end of the nation’s struggle with the pandemic may be coming into sight. But while the two currently approved Covid-19 vaccines from Pfizer-BioNTech and Moderna are more than 90 percent effective at preventing the development of serious illness, scientists don’t know whether someone who has been vaccinated can carry the live virus and spread it to others. Initial vaccine trials focused on vaccine safety. These were designed to gather data quickly and accurately on how effectively the vaccines prevented large groups of people from getting seriously sick with Covid-19. In the push to get a vaccine approved for emergency use as quickly as possible, other effects of the vaccines were left untested. Scientists must test a smaller pool of people with greater frequency to understand how the virus travels between people after vaccination—an effort that became secondary to studying vaccine safety and efficacy. “We design the trials to determine how we reduce the disease burden and keep people from progressing to hospitalization and death and being on a ventilator—that was and I think, still is, the first primary purpose of developing a vaccine,” says Larry Corey, co-director of the Covid-19 Prevention Network, a group formed in part by the National Institutes of Health to address the need for vaccines. Now, as new, highly contagious SARS-CoV-2 variants from California, the United Kingdom, South Africa and Brazil spread globally, understanding transmission as it relates to vaccine rollout efforts is vital. Most vaccines still seem to prevent worst outcomes, like hospitalization and death, against the new variants. However, it may be months before researchers have conclusive findings about how viral transmission from vaccinated individuals to unvaccinated individuals works. In the meantime, health experts recommend vaccinated people continue to adhere to current mask and social distancing practices. “You’re self-protected, but you still could be a danger to other people, especially if you start using behavioral disinhibition, saying, ‘I'm vaccinated, I'm invulnerable’,” Corey says. “You could acquire Covid and it will be silent, and then you can infect a bunch of people who are not as lucky as you to be vaccinated at this point in time.” The two approved mRNA vaccines provide systemic immunity, meaning they encourage the production of antibodies in the blood and trigger a whole-body response to the virus. However, the virus typically first infects the mucus of a person’s nose and mouth, where those antibodies don’t actively fend off pathogens. A study published in the Journal of Allergy and Clinical Immunology in November shows that people who recover from natural Covid-19 infections develop antibodies to protect the mucosal regions in the respiratory tract, but there is no evidence yet that the same is true with vaccine-induced immunity. Deborah Lehman, a professor of clinical pediatrics at UCLA, says if a Covid-19 vaccine is able to prevent the virus from living in the mucosal passages, it may not be able to spread to other people. Since scientists haven’t yet found evidence that the vaccines provide mucosal immunity, someone who is vaccinated and has no symptoms of illness may be carrying the live SARS-CoV-2 virus and spreading it to others when they cough, breath or sneeze. “You could have a lot of people vaccinated who are walking around but are still acquiring the virus—potentially still being infectious—and we don't really see a reduction on a population basis of disease burden,” Corey says. To test whether this population is spreading live virus, Corey says researchers need to collect samples from a large group of vaccinated people multiple times per week for evidence of viral shedding. Corey’s team at the Covid-Prevention Network (CoVPN) proposed a study of 20,000 vaccinated college students to track transmission on a campus; it’s still awaiting fe