Updated: December 11, 2020 10:59:45 am
Written by Apoorva Mandavilli
The new COVID-19 vaccines from Pfizer and Moderna appear to be very good at preventing serious diseases. But it is not clear how well they will slow the spread of the coronavirus.
That’s because the Pfizer and Moderna trials tracked only how many vaccinated people got sick with COVID-19. That leaves open the possibility that some vaccinated people become infected without developing symptoms and then can silently transmit the virus, especially if they come into close contact with other people or stop wearing masks.
If vaccinated people silently spread the virus, they can keep it circulating in their communities, putting unvaccinated people at risk.
“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 will really be critical for them to know if they have to keep wearing masks, 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 that remember the virus, quickly block the virus in the nose before it has a chance to take hold in other parts of the body.
Coronavirus vaccines, by contrast, 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 sufficient protection to prevent the vaccinated person from getting sick.
Some of these antibodies will circulate through the nasal mucosa and be on guard there, but it is not clear how many antibodies can be mobilized or how quickly. If the answer is not much, viruses could flourish in the nose and be sneezed or exhaled to infect others.
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“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. “This is a really important question.”
This is why mucosal vaccines, such as FluMist nasal spray or oral polio vaccine, are better than intramuscular injections in defending against respiratory viruses, experts said.
The next generation of coronavirus vaccines can elicit immunity in the nose and the rest of the respiratory tract, where it is most needed. Or people could receive an intramuscular injection followed by a mucosal boost that produces protective antibodies in the nose and throat.
Coronavirus vaccines have proven to be powerful shields against serious illness, but that does not guarantee their effectiveness in the nose. The lungs, the site of severe symptoms, are much more accessible to circulating antibodies than the nose or throat, making them easier to protect. 📣 Follow Express explained on Telegram
“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 is 95% effective in preventing symptomatic illnesses, it will surely be a little less than that in preventing all infections.”
Still, he and other experts said they were optimistic 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 from the vaccine, your ability to get 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 must be reduced.”
The vaccine trials did not provide data on how many vaccinated people were infected with the virus but had no symptoms. However, some clues are emerging.
AstraZeneca, which announced some of the results of its trial in November, said that volunteers had been regularly screened for the virus and that those results suggested the vaccine might prevent some infections.
Pfizer will screen a subset of its trial participants for antibodies to a viral protein called N. Because vaccines have nothing to do with this protein, N antibodies would reveal whether volunteers had become infected with the virus afterward. of immunization, said Jerica Pitts, a company spokeswoman.
Moderna also plans to analyze the blood of all its participants and perform N antibody tests.
“It will be several weeks before we can expect to see those results,” said Colleen Hussey, a spokeswoman for Moderna.
So far, trials have only looked at blood, but mucosal antibody tests would confirm that antibodies can travel to the nose and mouth. Tal’s team is planning to test blood and saliva samples from volunteers in the Johnson & Johnson trial to see how the two antibody levels compare.
Meanwhile, Bhattacharya said, he was encouraged by recent work showing that people who received an intramuscular flu vaccine had abundant antibodies in their noses. And a study of COVID-19 patients found that antibody levels in saliva and blood closely matched, suggesting 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.
But some studies have suggested that even people without symptoms can have large amounts of coronavirus in their noses, noted Dr. 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 was harboring enough virus to infect others.
Vaccinated people who have a high viral load but have no symptoms “would actually be, in some ways, even worse spreaders because they may be under a false sense of security,” Maldonado said.
Tal said she was concerned about studies with monkeys showing that some vaccinated animals did not get sick, but still had viruses in their noses.
But those monkeys were intentionally exposed to massive amounts of virus and still had less virus than unvaccinated animals, said John Moore, a virus expert at Weill Cornell Medicine in New York.
“The more you lower your viral load, the less likely you are to be transmissible,” Moore said. But “these are all things where data trumps theory, and we need the data.”
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