According to the FDA, many vaccines that protect against viral illnesses might also reduce transmission. In a February 2021 pre-print article, scientists looked at transmission among people who received the Pfizer COVID-19 vaccine and then subsequently became infected with the virus. The scientists found viral loads — which can indicate contagiousness — to be lower in patients who became infected 12-28 after their first shot. This may hint that the vaccine could lower infectiousness and impact the spread of COVID-19. Trials of the Moderna vaccine and the Oxford-AstraZeneca vaccines suggested it might be possible that the vaccines prevent people from being infected entirely (rather than just reducing the severity of symptoms), but more validation is needed. The exact amount of reduction in transmission as a result of vaccination is still unknown. It is also unclear how long the protection lasts. Additionally, the present vaccines are reported to have about 95% efficacy. This means that about 5% of people might not be protected, despite being vaccinated. The U.S. C.D.C. and public health professionals advise wearing a mask (though the group now recommends wearing a cloth mask over a surgical mask when not wearing disposable masks or KN95/N95 masks) and maintaining physical distance even after getting vaccinated, in order to protect those that are not vaccinated as yet and to stop further spread in the community.

The flu vaccine is another example where there is less chance for people who are vaccinated to get the flu or fall severely ill, but they are capable of spreading influenza around in the community