One question looms large as people across the world grapple with the prospect of living with the coronaviruses for the foreseeable future, how soon before they need another shot?
Not for a long time, according to a flurry of new studies.
Studies suggest that three or even two Covid vaccine doses are enough to protect most people from serious illness and death for a long time.
John Wherry is the director of the Institute for Immunology at the University of Pennsylvania. He said that it may be unnecessary for most people to get a fourth vaccine dose.
The Biden administration's top Covid adviser said that they are unlikely to recommend a fourth dose before the fall.
After two doses of a Covid vaccine, the Omicron variant can dodge the immune molecules that prevent the virus from infecting cells. A third shot of the vaccine made by Pfizer-BioNTech or Moderna causes the body to make a much wider variety of antibodies, which would be difficult for any variant of the virus to evade, according to a study posted online on Tuesday.
The study suggests that people should be protected from new versions of the virus even if they differ from the original version.
If people are exposed to another variant like Omicron, they now have more options to fight it.
According to four studies published in top-tier journals over the past month, other parts of the immune system can remember and destroy the virus for many months if not years.
The research shows that the T cells produced by the four brands of Covid vaccine are 80 percent more powerful against Omicron than any other variant. Researchers said it was very likely that T cells would mount a similar attack on any future variant.
This is similar to what scientists have found in the case of the Severe Acute Respiratory Syndrome coronaviruses. T cells have lasted more than 17 years in people who have been exposed to that virus. Experts said that the immune cells for the new coronaviruses may decline very slowly.
Wendy Burgers, an immunologist at the University of Cape Town who led one of the studies, said that the T-cell response can last for ages.
The body's first line of defense against a virus has been the focus of a disproportionate amount of research. The molecule can be measured from a drop of blood.
Analyzing immune cells requires a lot of time and a lot of blood.
Few labs have the ability to study these cells, and their findings lag weeks behind those on antibodies. Scientists have often overlooked the importance of other parts of the immune system.
A lot of doctors and scientists don't know what a T cell is.
T cells are more important than people think, according to Dr. Barouch.
After every vaccine shot or exposure to the virus, the antibodies spike and decline in a few weeks.
Federal officials recommend boosters for everyone older than 12 years old after a decline in antibody levels. According to recent data from the Centers for Disease Control and Prevention, the extra shots seem to lose some of their ability to prevent infections within four months.
Two or three key parts of the spikeProtein, a protrusion on the outside of the coronaviruses that allows it to latch on to human cells, are recognized by antibodies. T cells can detect more parts of the spike and are less likely to fail when the virus gains some of them.
The B cells of the immune system can make fresh batches of antibodies within four or five days after a new exposure to the virus.
Many people who received two or even three doses of vaccine could still be affected by the Omicron variant, but only a small percentage became seriously ill.
If memory B cells are still there, and memory T cells are still there, they can kick back into action relatively quickly.
Over time, memory B cells learn to recognize a diverse set of viral genetic sequences. The longer they have to practice, the more they can stop the virus.
The germinal center, where B cells train, remains active for at least 15 weeks after the second dose of a Covid vaccine. In an updated study published in the journal Nature, the same team showed that six months after vaccination, memory B cells continue to mature, and the antibodies they produce keep gaining the ability to recognize new variant.
The study found that the antibodies produced one month after immunization are more potent than the ones produced six months later.
A third shot creates an even richer pool of B cells than the second shot did, and the antibodies they produce recognize a broader range of variants, according to a new study. These antibodies were able to fight off the Delta and Omicron variant in laboratory experiments. Even though the vaccine was not designed for that variant, more than half of the antibodies seen one month after a third dose were able to neutralizing Omicron.
If you have had a third dose, you will have a rapid response that is specific to Omicron.
According to a study published last month in Nature Medicine, memory cells produced after the coronaviruses seem less potent against the Omicron variant. The vaccine response is much more consistently good than the immune system's response, according to Marcus Buggert, an immunologist at the Karolinska Institute in Sweden who led the study.
Dr. Buggert said that about one in five people had a significant reduction in their T cell response against Omicron. He said that the differences are most likely due to their genetics.
Recent studies show that most people will hold up their immunity from infections for a long time. Experts said there would still be enough others to maintain a strong immune response even if the new variant changed some of the viral regions that T cells recognize.
One big unknown is how slowly the T cells may decline, and whether two doses of vaccine can create a long- lasting response, or if instead people would need three.
That is a question that we don't know the answer to.
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