The new Covid-19 vaccines from Pfizer and Moderna seem to be remarkably good at preventing serious illness. But it’s unclear how well they will curb the spread of the coronavirus.

That’s because the Pfizer and Moderna trials tracked only how many vaccinated people became sick with Covid-19. That leaves open the possibility that some vaccinated people get infected without developing symptoms, and could then silently transmit the virus — especially if they come in close contact with others or stop wearing masks.

If vaccinated people are silent spreaders of the virus, they may keep it circulating in their communities, putting unvaccinated people at risk.

“A lot of people are thinking that once they get vaccinated, they’re not going to have to wear masks anymore,” said Michal Tal, an immunologist at Stanford University. “It’s really going to be critical for them to know if they have to keep wearing masks, because they could still be contagious.”

In most respiratory infections, including the new coronavirus, the nose is the main port of entry. The virus rapidly multiplies there, jolting the immune system to produce a type of antibodies that are specific to mucosa, the moist tissue lining the nose, mouth, lungs and stomach. If the same person is exposed to the virus a second time, those antibodies, as well as immune cells that remember the virus, rapidly shut down the virus in the nose before it gets a chance to take hold elsewhere in the body.

The coronavirus vaccines, in contrast, are injected deep into the muscles and quickly absorbed into the blood, where they stimulate the immune system to produce antibodies. This appears to be enough protection to keep the vaccinated person from getting ill.

Some of those antibodies will circulate to the nasal mucosa and stand guard there, but it’s not clear how much of the antibody pool can be mobilized, or how quickly. If the answer is not much, then viruses could bloom in the nose — and be sneezed or breathed out to infect others.

“It’s a race: It depends whether the virus can replicate faster, or the immune system can control it faster,” said Marion Pepper, an immunologist at the University of Washington in Seattle. “It’s a really important question.”

This is why mucosal vaccines, like the nasal spray FluMist or the oral polio vaccine, are better than intramuscular injections at fending off respiratory viruses, experts said.

The next generation of coronavirus vaccines may elicit immunity in the nose and the rest of the respiratory tract, where it’s most needed. Or people could get an intramuscular injection followed by a mucosal boost that produces protective antibodies in the nose and throat.

The coronavirus vaccines have proved to be powerful shields against severe illness, but that is no guarantee of their efficacy in the nose. The lungs — the site of severe symptoms — are much more accessible to the circulating antibodies than the nose or throat, making them easier to safeguard.

“Preventing severe disease is easiest, preventing mild disease is harder, and preventing all infections is the hardest,” said Deepta Bhattacharya, an immunologist at the University of Arizona. “If it’s 95 percent effective at preventing symptomatic disease, it’s going to be something less than that in preventing all infections, for sure.”

Still, he and other experts said they were optimistic that the vaccines would suppress the virus enough even in the nose and throat to prevent immunized people from spreading it to others.

“My feeling is that once you develop some form of immunity with the vaccine, your ability to get infected will also go down,” said Akiko Iwasaki, an immunologist at Yale University. “Even if you’re infected, the level of virus that you replicate in your nose should be reduced.”

The vaccine trials have not produced data on how many vaccinated people were infected with the virus but did not have symptoms. Some hints are emerging, however.

AstraZeneca, which announced some of its trial results in November, said that volunteers had been testing themselves regularly for the virus, and that those results suggested that the vaccine might prevent some infections.

Pfizer will test a subset of its trial participants for antibodies against a viral protein called N. Because the vaccines have nothing to do with this protein, N antibodies would reveal whether the volunteers had become infected with the virus after immunization, said Jerica Pitts, a spokeswoman for the company.

The Coronavirus Outbreak ›

Words to Know About Testing

Confused by the terms about coronavirus testing? Let us help:

    • Antibody: A protein produced by the immune system that can recognize and attach precisely to specific kinds of viruses, bacteria, or other invaders.
    • Antibody test/serology test: A test that detects antibodies specific to the coronavirus. Antibodies begin to appear in the blood about a week after the coronavirus has infected the body. Because antibodies take so long to develop, an antibody test can’t reliably diagnose an ongoing infection. But it can identify people who have been exposed to the coronavirus in the past.
    • Antigen test: This test detects bits of coronavirus proteins called antigens. Antigen tests are fast, taking as little as five minutes, but are less accurate than tests that detect genetic material from the virus.
    • Coronavirus: Any virus that belongs to the Orthocoronavirinae family of viruses. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
    • Covid-19: The disease caused by the new coronavirus. The name is short for coronavirus disease 2019.
    • Isolation and quarantine: Isolation is the separation of people who know they are sick with a contagious disease from those who are not sick. Quarantine refers to restricting the movement of people who have been exposed to a virus.
    • Nasopharyngeal swab: A long, flexible stick, tipped with a soft swab, that is inserted deep into the nose to get samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be collected with swabs that do not go as deep into the nose — sometimes called nasal swabs — or oral or throat swabs.
    • Polymerase Chain Reaction (PCR): Scientists use PCR to make millions of copies of genetic material in a sample. Tests that use PCR enable researchers to detect the coronavirus even when it is scarce.
    • Viral load: The amount of virus in a person’s body. In people infected by the coronavirus, the viral load may peak before they start to show symptoms, if symptoms appear at all.

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