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 federal funding. Lehman says studying the viral load in vaccinated people can help researchers understand how infectious they are compared to non-vaccinated people.

Given the rate of vaccinations, the duration of testing, and quantity of samples needed, Corey and Lehman expect researchers won’t collect enough data on transmission to have an answer until the fall. Having more information about virus transmission is crucial to the future of informed public health recommendations. If vaccinated people can still spread the virus, it could change the timeline for reopening businesses, allowing large gatherings and loosening current restrictions.

The Pfizer-BioNTech vaccine doesn’t provide significant immune protection until 12 days after the first dose and only reaches 52 percent efficacy after a few weeks, per a study published in The New England Journal of Medicine in December. The Moderna vaccine is similarly 51 percent effective two weeks after the first immunization, per its application for authorization.

During this time, the body is still relatively vulnerable to infection. People will need to be mindful of when their friends and family got vaccinated in order to understand their immunity status, which will get complicated over time as more of the population gets vaccines.

“Vaccination hubs and centers are reinforcing the information that after the first dose and after the second dose you need to continue to practice these public health measures,” Lehman says. “[Immunity] takes a while and I think that's true for all vaccines.”

Ann Marie Pettis, who leads a national organization of infection preventionists, says experts are working to provide the most up-to-date Covid-19 information to the research community and general public so people can make safe decisions.

“There're so many more questions than answers, unfortunately,” Pettis says. “You just have to stay in touch with the data and with the science and try to keep track of what the experts are coming up with, from day to day.”

Until scientists are certain about the risks of transmission, and a large enough portion of the population is vaccinated to achieve herd immunity, Pettis says all people must continue to wear masks, practice social distancing and maintain good hygiene.

While widespread vaccination is a major milestone in combatting the Covid-19 pandemic, Lehman says it’s no “magic bullet.” Until more information becomes available, people should continue to live, work and travel with an abundance of caution for public health.

“The vaccine gives us all a certain amount of comfort, which is good, but I think it would be a mistake to just assume, get two vaccines and then we can have large gatherings again,” Lehman says. “It’s going to be a while before we feel comfortable recommending that all those restrictions be relaxed.”

Passengers must be tested with a testing.html">viral test that could be either an antigen test or a nucleic acid amplification test (NAAT). Available NAATs for SARS-CoV-2 include reverse transcription polymerase chain reaction (RT-PCR), reverse transcription loop-mediated isothermal amplification (RT-LAMP), transcription-mediated amplification (TMA), nicking enzyme amplification reaction (NEAR), and helicase-dependent amplification (HDA). The test used must be authorized for use by the relevant national authority for the detection of SARS-CoV-2 in the country where the test is administered. A viral test conducted for U.S. Department of Defense (DOD) personnel, including DOD contractors, dependents, and other U.S. government employees, and tested by a DOD laboratory located in a foreign country also meets the requirements of the Order.

Rapid tests are acceptable as long as they are a viral test acceptable under the Order.

The Order requires a lab report to be presented to the airline or to public health officials upon request. A home specimen collection kit that is tested in a laboratory should meet the requirements, if such methods have been authorized by the country’s national health authorities. A viral test conducted for U.S. Department of Defense (DOD) personnel, including DOD contractors, dependents, and other U.S. government employees, and tested by a DOD laboratory located in a foreign country also meets the requirements of the Order.