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It’s surprising, but experts agree: The tens of thousands of people being vaccinated against the coronavirus right now may still be able to carry and transmit the SARS-CoV-2 virus to others, despite being well-protected against the symptoms of COVID-19 themselves. That means they must still wear masks and practice social distancing to protect those around them.

“We still have yet to learn how much infectivity is impacted by vaccination, thereby producing protection for those I am in contact with, not just for myself, when I receive a vaccine,” says Dr. Janet Baseman, a public health expert at the University of Washington.

This uncertainty is a consequence of the extremely rapid vaccine development process. Normally, a vaccine trial would observe recipients of a vaccine for a longer period of time, showing not just whether they became ill, but whether they spread contagion, before a vaccine entered wide use. While it’s likely that the various coronavirus vaccine candidates provide at least some reduction in transmission, it’s simply too soon to know for sure, or to what degree.

The science behind this seeming contradiction is relatively straightforward. Vaccines like Pfizer’s and Moderna’s are injected into deep muscle and provide so-called systemic immunity. But they may still leave areas, particularly nasal mucous passages, where the virus is relatively free to multiply. Even without making the host sick, that virus can then potentially spread through coughing or sneezing, just as it is spread by a person with a full-blown coronavirus infection.

“Having systemic immunity is like having a security guard that’s somewhere in the building,” says Joshua Michaud, an epidemiologist at KFF (formerly the Kaiser Family Foundation). “Having mucosal immunity is like having a security guard right at the door, ready to take on the incoming virus.”

It could take researchers months to determine just where that viral security guard is stationed, because it requires monitoring people who have already been vaccinated. According to Baseman, a study focusing on vaccinated people “with high levels of mixing and high risk of transmission,” such as college students, could provide faster data on post-vaccination transmission. Michaud expects focused transmission studies to be conducted in "the coming weeks and months."

Insight on the question will also come through ongoing vaccine trials. Though both Pfizer’s and Moderna’s vaccines have been approved for emergency use by health authorities such as the U.S. FDA, the drugmakers are continuing the standard trials that should lead to permanent approval.

“Our trial will continue to determine the full protection and potential of the vaccine,” including transmission after vaccination, according to a Pfizer spokesperson.

But until long-term trial data or other experimental results are released, post-vaccine infectivity will be made clear only by measuring the spread of the coronavirus through populations with high vaccination levels. If the spread slows, it will demonstrate that transmission is lowered by vaccination. But it is expected to take much of 2021 to reach high levels of vaccination even in countries at the front of the line, so that approach could take the better part of a year.

According to the FDA, most vaccines that protect from viral illnesses also reduce viral transmission by those who are vaccinated, and there is reason for optimism that the current wave of COVID vaccines will, as well. Most notably, Moderna’s vaccine trial showed some reduction in asymptomatic infections as measured through nasal swabs, which could suggest reduced transmission after vaccination. Dr. Eric Topol of the Scripps Research Translational Institute has described those findings as inconclusive, but promising on the question of post-vaccine transmission.

Until more conclusive data arrives, though, the message from health authorities is clear: Even after you've been vaccinated, don't toss that face mask.