MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer.
After getting two doses of the COVID-19 vaccine, will I be able to carry the virus even if I don’t become ill from it? Will I still need to mask and physically distance from other people?
Yes, you will need to continue wearing a mask and taking other precautions after being fully vaccinated. That’s because it’s possible that vaccinated people will still be able to spread the virus, even if they do not develop symptoms themselves.
There are several ways to demonstrate a vaccine’s effectiveness. We can show that it prevents infection, prevents disease, or prevents transmission — ideally, of course, all of the above. But when it comes to the two COVID-19 vaccines currently authorized for emergency use in the United States, we’re not there yet.
Clinical trials have already demonstrated that both Pfizer and Moderna vaccines are really good at the second function, preventing disease. In Pfizer’s clinical trial, for example, researchers gave 43,661 volunteers either the vaccine or a placebo and then waited for people to develop symptoms of COVID-19. They tested any participant who came down with even mild symptoms, and 170 tested positive. Of these, 162 were in the placebo group; just eight had received the actual vaccine. A statistical comparison of the relative difference between the fraction of individuals in each group who had gotten sick produced a vaccine efficacy value of 95 percent. Through a similar process, Moderna calculated the efficacy of their vaccine at 94.1 percent.
In short, the evidence strongly indicates that both vaccines do a good job of keeping people from becoming ill with COVID-19. However, both Pfizer and Moderna trials tested only participants who developed symptoms. In other words, it is entirely possible that some vaccinated participants became infected and able to transmit the virus, even if they didn’t develop any of the symptoms that would have prompted testing and diagnosis.
Indeed, some immunologists have theorized that intramuscular vaccines, which spur production of antibodies in the blood, may not generate a particularly rapid immune response in the mucosa, the moist tissue lining the nose, which is where the SARS-CoV-2 virus normally enters the body. This, they say, could allow the virus to multiply in the nose of a vaccinated person for some period of time — even if the individual doesn’t become ill. On the other hand, a 2015 study of an intramuscular influenza vaccine showed that vaccinated individuals had abundant nasal antibodies. Additionally, a limited analysis of swabs taken between volunteers’ first and second shots in the Moderna trial indicated that even a single dose of the vaccine might offer some level of protection against infection.
The question of whether these vaccines will prevent infection or transmission is not a trivial one. Current CDC estimates are that 40 percent of infected people never develop symptoms but are nonetheless capable of spreading the virus. If vaccinated people can become infected without developing symptoms, they could join the ranks of these silent spreaders, contributing to greater community transmission of the virus and endangering the health of people who have not yet received their vaccines.
As we begin to vaccinate the first members of our own community, we are introducing a potent and tangible line of defense in the fight against COVID-19. Researchers are working to determine the extent of the protection the vaccine confers. In the meantime, all of us, vaccinated or not, must continue to use the other potent and tangible defenses we possess, including our humble facemasks. By continuing to take these precautions, we will be doing something even more powerful — protecting others.
