The new vaccines for COVID-19 from Pfizer and Moderna appear to be extraordinarily good at preventing the severe version of the disease. But it is unclear how well they will slow the spread of the coronavirus.
That’s because the Pfizer and Moderna tests only tracked how many vaccinated people got sick from COVID-19. That leaves open the possibility that some vaccinated people become infected without developing symptoms, and may then silently transmit the virus, especially if they come into close contact with others or stop wearing a mask.
If vaccinated people are silent carriers of the virus, they can keep it circulating in their communities and thus put unvaccinated people at risk.
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–“A lot of people think that once they get vaccinated they won’t have to wear masks anymore,” said Michal Tal, an immunologist at Stanford University. “It’s going to be really critical for them to know if they have to keep wearing a mask, because they could still be contagious.”
For most respiratory infections, including the new coronavirus, the nose is the main port of entry. The virus multiplies rapidly there, jolting the immune system to produce a class of antibodies that are specific to the mucosa, the moist tissue that lines 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 reminiscent of the virus, quickly kill the virus in the nose before it has a chance to take root in another part of the body.
Instead, coronavirus vaccines are injected deep into the muscles and rapidly absorbed into the blood, where they stimulate the immune system to produce antibodies. This appears to be enough protection to prevent the vaccinated person from getting sick.
Some of these antibodies will circulate through the nasal mucosa and stand guard there, but it is not clear how much of the antibody pool it can be mobilized, or how quickly. If the answer is not much, then the viruses could flourish in the nose and be sneezed or exhaled to infect others.
“It’s a race – it depends on whether the virus can replicate faster or whether the immune system can control it faster,” said Marion Pepper, an immunologist at the University of Washington in Seattle. “It is a very important question.”
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–Coronavirus vaccines have proven to be powerful shields against the severe version of the disease, but that is no guarantee of their effectiveness in the nose. The lungs – the scene of severe symptoms – are much more accessible to circulating antibodies than the nose or throat, making them easier to safeguard.
“Preventing a serious illness is easier, preventing a mild illness is more difficult, and preventing all infections is the hardest,” said Deepta Bhattacharya, an immunologist at the University of Arizona. “If it’s 95 percent effective in preventing symptomatic disease, it will be a little less than that in preventing all infections, for sure.”
Still, he and other experts said they were optimistic the vaccines would suppress the virus enough, including 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, the ability to become infected will also decrease,” said Akiko Iwasaki, an immunologist at Yale University. “Even if you are infected, the level of virus replicating in your nose should be lower.”
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–Vaccine trials have not produced data on how many vaccinated people were infected with the virus but had no symptoms. However, some signs have emerged.
AstraZeneca, which announced some of the results of his trials in November, he said the volunteers had been regularly tested for the virus, and that those results suggested the vaccine might prevent some infections.
The Coronavirus Outbreak ›
Words to Know About Testing
Confused by the terms about coronavirus testing? Let us help:
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- 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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Pfizer will test a subset of its trial participants for antibodies against a viral protein called N. Because vaccines have nothing to do with this protein, N antibodies would reveal whether volunteers had been infected with the virus afterward. of immunization, said Jerica Pitts, a company spokeswoman.
Moderna also plans to test the blood of all its participants and do tests for N antibodies. “It will be several weeks before we can expect to see those results,” said Colleen Hussey, a spokeswoman for Moderna.
So far, tests have only looked at blood, but mucosal antibody analysis would confirm that antibodies can travel to the nose and mouth. Tal’s team plans analyze blood and saliva samples of volunteers from the Johnson & Johnson trial to see how the two antibody levels compare.
Meanwhile, Bhattacharya said, he was encouraged by the recent job showing that people who got a flu shot intramuscularly had abundant antibodies in their noses. And a study of COVID-19 patients found that levels of antibodies in saliva and blood were closely paired, which suggests that a strong immune response in the blood would also protect mucosal tissues.
Only people who have the virus in their nose and throat would be expected to transmit the virus, and the lack of symptoms in immunized people who became infected suggests that the vaccine may have kept virus levels in check.
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–But some studies have suggested that even people without symptoms can have high amounts of coronavirus in their nose, said Yvonne Maldonado, who represents the American Academy of Pediatrics at meetings of the Federal Advisory Committee on Immunization Practices. The first person confirmed to be reinfected with the coronavirus, a 33-year-old man in Hong Kong, also had no symptoms, but harbored enough virus to infect others.
Vaccinated people who have a high viral load but have no symptoms “would actually be, in some way, even worse spreaders because they could be under a false sense of security,” Maldonado said.
Dr. Tal said she was concerned about studies on monkeys that showed that some vaccinated animals did not get sick, but still had the virus in their noses.
But those monkeys were intentionally exposed to huge amounts of viruses and still had less virus than unvaccinated animalssaid John Moore, a virologist at Weill Cornell Medicine in New York.
“The more the viral load is lowered, the less likely it is to be transmitted,” Moore said. But “these are all things where data beats theory, and we need the data.”
Apoorva Mandavilli is a reporter for the Times and focuses on science and global health. In 2019 he won the Victor Cohn Award for Excellence in Medical Science Reporting. @apoorva_nyc
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