Pfizer and Moderna's new Covid-19 vaccines appear remarkably good at preventing serious diseases. However, it is unclear how well they will contain the spread of the coronavirus.
This is because the Pfizer and Moderna studies only recorded how many vaccinated people were infected with Covid-19. This leaves the possibility open that some vaccinated people could become infected without developing symptoms and then quietly 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 can keep it circulating in their communities and put unvaccinated people at risk.
"A lot of people think they don't have to wear masks after vaccination," said Michal Tal, an immunologist at Stanford University. "It's going to be really important that you know if you need to keep wearing masks because they can still be contagious."
For most respiratory infections, including the new coronavirus, the nose is the main port of entry. The virus multiplies quickly there, shaking the immune system to produce a type of antibody that is specific to the mucous membrane, with the moist tissue lining the nose, mouth, lungs and stomach. When the same person is exposed to the virus a second time, these antibodies, as well as immune cells that remember the virus, quickly turn off the virus in the nose before it can gain a foothold elsewhere in the body.
In contrast, the coronavirus vaccines are injected deep into the muscles and quickly absorbed into the blood, where they stimulate the immune system to produce antibodies. This seems to be sufficient protection to keep the vaccinated person from disease.
Some of these antibodies circulate to the nasal mucosa and stand guard there, but it is not clear how much of the antibody pool can be mobilized or how quickly. If the answer isn't much, viruses can bloom in the nose – and sneeze or exhale 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's a really important question."
For this reason, mucosal vaccines such as the FluMist nasal spray or the oral polio vaccine are better than intramuscular injections to ward off respiratory viruses, according to experts.
The coronavirus vaccines have been shown to be effective shields against serious illness, but that is no guarantee of their effectiveness in the nose. The lungs – the location of severe symptoms – are much more accessible to circulating antibodies than the nose or throat, making them easier to protect.
"Preventing major diseases is the easiest, preventing mild diseases is more difficult, and preventing all infections is the most difficult," said Deepta Bhattacharya, an immunologist at the University of Arizona. "If it's 95 percent effective at preventing symptomatic disease, it will certainly be less than that at 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 immunized people from spreading it to others.
"I have a feeling that once you develop some form of immunity with the vaccine, your ability to become infected will decrease," said Akiko Iwasaki, an immunologist at Yale University. "Even if you are infected, the level of the virus that you replicate in your nose should be reduced."
The vaccine trials did not provide any data on how many people who were vaccinated were infected with the virus, but they did not have any symptoms. There are some pointers, however.
AstraZeneca, which announced some of its trial results in November, said volunteers had tested themselves regularly for the virus and that those results suggested the vaccine might prevent some infections.
Pfizer will test a subset of its study participants for antibodies to a viral protein called N. Since the vaccines have nothing to do with this protein, N-antibodies would show whether the volunteers became infected with the virus after immunization, said Jerica Pitts, a company spokeswoman.
The coronavirus outbreak>
Things to know about testing
Confused by Coronavirus Testing Conditions? Let us help:
- antibody: A protein produced by the immune system that can recognize and bind to certain types of viruses, bacteria or other invaders.
- Antibody test / serology test: A test that detects antibodies specific to the coronavirus. About a week after the coronavirus infects the body, antibodies begin to appear in the blood. Because antibodies take so long to develop, an antibody test cannot reliably diagnose an ongoing infection. However, it can identify people who have been exposed to the coronavirus in the past.
- Antigen test: This test detects parts of coronavirus proteins called antigens. Antigen tests are quick and only take five minutes. However, they are less accurate than tests that detect genetic material from the virus.
- Coronavirus: Any virus that belongs to the Orthocoronavirinae virus family. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
- Covid19: The disease caused by the new coronavirus. The name stands for Coronavirus Disease 2019.
- Isolation and quarantine: Isolation is separating people who know they have 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 smear: A long, flexible rod with a soft swab that is inserted deep into the nose to collect samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be taken with swabs that don't go as deep into the nose – sometimes called nasal swabs – or with swabs from the mouth or throat.
- Polymerase chain reaction (PCR): Scientists use PCR to make millions of copies of genetic material in a sample. With the help of PCR tests, researchers can detect the coronavirus even when it is scarce.
- Viral load: The amount of virus in a person's body. In people infected with the coronavirus, viral loads can peak before symptoms, if any.
Moderna also plans to analyze the blood of all participants and test for N-antibodies. "It will be a few weeks before we can expect these results," said Colleen Hussey, a spokeswoman for Moderna.
So far, the studies have only analyzed blood, but tests for antibodies in the mucous membrane would confirm that the antibodies can get into the nose and mouth. The team of Dr. Tal plans to run matching blood and saliva samples from volunteers in the Johnson & Johnson study to see how the two antibody levels compare.
In the meantime, Dr. Bhattacharya, he was encouraged by recent work showing that people who were given an intramuscular flu vaccine had antibodies in their noses in abundance. A study of Covid-19 patients found that antibody levels in saliva and blood were closely matched – suggesting that a strong immune response in the blood would also protect the mucosal tissue.
Only people with the virus swarming their noses and throats are expected to transmit the virus. The absence of symptoms in the infected immunized individuals suggests that the vaccine may have kept virus levels in check.
However, some studies have shown that even people with no symptoms can have high levels of coronavirus in their nose, noted Dr. Yvonne Maldonado, who represents the American Academy of Pediatrics at meetings of the Federal Advisory Board on Immunization Practices. The first person confirmed to be 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 with a high viral load but no symptoms "would in a way be even worse spreaders because they may be under a false sense of security," said Dr. Maldonado.
Dr. Tal said she was concerned about monkey studies that showed some vaccinated animals did not get sick but still had viruses in their noses.
But these monkeys were purposely exposed to massive amounts of virus and still had fewer viruses than non-vaccinated animals, said John Moore, a virologist at Weill Cornell Medicine in New York.
"The more you reduce the viral load, the less likely you are to be communicable," said Dr. Moors. But "all of these are things where data trumps theory and we need the data."