Pfizer and Moderna’s new Covid-19 vaccines appear remarkably effective in preventing serious illness. But it is not known to what extent they will curb the spread of the coronavirus.
Indeed, the Pfizer and Moderna trials only tracked the number of vaccinated people who fell ill with Covid-19. This leaves open the possibility that some vaccinated people will be infected without developing symptoms and can then silently transmit the virus – especially if they come into close contact with others 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 vaccinated they won’t have to wear a mask anymore,” said Michal Tal, an immunologist at Stanford University. “It will be really essential for them to know if they should continue to wear masks because they could still be contagious.”
In most respiratory infections, including the novel coronavirus, the nose is the primary port of entry. The virus multiplies quickly there, shaking the immune system to produce a type of specific antibody in the mucous membrane, the moist tissue lining the nose, mouth, lungs and stomach. If the same person is exposed to the virus a second time, these antibodies, along with the immune cells that remember the virus, quickly block the virus in the nose before it has a chance to settle elsewhere in the body. .
Vaccines against the coronavirus, on the other hand, are injected deep into the muscles and quickly absorbed into the bloodstream, 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 travel to and guard the nasal mucosa there, but it is not known how much of the antibody pool can be mobilized or how quickly. If the answer isn’t much, then viruses could flourish in your nose – and be sneezed or exhaled to infect others.
“It’s a race: it depends if the virus can replicate faster or if 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.”
That’s why mucosal vaccines, like FluMist nasal spray or oral polio vaccine, are better than intramuscular injections at repelling respiratory viruses, experts said.
Coronavirus vaccines have been shown to be powerful shields against serious disease, 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.
“Prevention of serious illness is the easiest, prevention of mild illness is more difficult, and prevention of all infections is the most difficult,” said Deepta Bhattacharya, an immunologist at the University of Arizona. “If it’s 95% effective in preventing symptomatic disease, it will certainly be less than that in preventing all infections.”
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 people with immunity from passing it on to others.
“My feeling is that once you develop some form of immunity with the vaccine, your ability to get infected will also decrease,” said Akiko Iwasaki, immunologist at Yale University. “Even if you are infected, the level of virus that you replicate in your nose should be reduced.”
Vaccine trials have not produced data on the number of vaccinated people infected with the virus but have not shown symptoms. A few clues emerge, however.
AstraZeneca, which announced some of its test results in November, said volunteers regularly tested themselves for the virus and the results suggested the vaccine could prevent some infections.
Pfizer will test 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 will reveal whether volunteers have been infected with the virus. after the vaccination, said Jerica Pitts, spokesperson for the company.
Confused by the terms relating to coronavirus testing? Let us help you:
- Antibody: A protein produced by the immune system that can recognize and attach specifically to specific types of viruses, bacteria, or other invaders.
- Antibody test / serological test: A test that detects specific antibodies to the coronavirus. Antibodies start 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 cannot 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 pieces of coronavirus protein called antigens. Antigen tests are quick, take only five minutes, but are less accurate than tests that detect the genetic material of the virus.
- Coronavirus: Any virus belonging to the Orthocoronavirinae family of viruses. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
- Covid19: The disease caused by the new coronavirus. The name is the abbreviation for coronavirus disease 2019.
- Isolation and quarantine: Isolation is the separation of people who know they have a contagious disease from those who do not. Quarantine refers to restricting the movement of people who have been exposed to a virus.
- Nasopharyngeal swab: A long, flexible stick with a soft swab that is inserted deep into the nose to take samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be collected with swabs that do not penetrate 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 using PCR allow researchers to detect the coronavirus even when it is rare.
- Viral load: The amount of virus in a person’s body. In people infected with the coronavirus, the viral load may peak before starting to show symptoms, if symptoms do appear.
Moderna also plans to analyze the blood of all of its participants and test for N antibodies. “It will be several weeks before we can expect to see these results,” said Colleen Hussey, spokesperson for Moderna.
So far, tests have only analyzed blood, but looking for antibodies in the lining would confirm that the antibodies can travel to the nose and mouth. Dr Tal’s team plans to analyze matched blood and saliva samples from volunteers in the Johnson & Johnson trial to see how the two levels of antibodies compare.
In the meantime, Dr Bhattacharya said, he was encouraged by recent work showing that people who had received an intramuscular influenza vaccine had abundant antibodies in their noses. And a study of patients with Covid-19 found that the levels of antibodies in saliva and blood were closely related, suggesting that a strong immune response in the blood would also protect mucous tissue.
Only people who have the virus swarming in their nose and throat are expected to pass the virus on, and the absence of symptoms in immune people who were infected suggests that the vaccine may have controlled levels of the virus.
But some studies have suggested that even people without symptoms can have large amounts of coronavirus in their nose, 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 have been re-infected with the coronavirus, a 33-year-old man in Hong Kong, also had no symptoms but contained enough virus to infect others.
Vaccinated people who have a high viral load but do not have symptoms “would in fact be, in some ways, even worse spreaders because they may be under a false sense of security,” said Dr Maldonado.
Dr Tal said she was concerned about studies in monkeys showing that some vaccinated animals did not get sick, but still had a virus in their nose.
But these monkeys were intentionally exposed to massive amounts of the virus and always had fewer viruses than unvaccinated animals, said John Moore, a virologist at Weill Cornell Medicine in New York City.
“The more you lower the viral load, the less likely you are to be transmissible,” said Dr. Moore. But “all of these things are things where the data trumps theory, and we need the data.