Covid-19, Reopening and Mask Mandate News: Live Updates

As people across the world grapple with the prospect of living with the coronavirus for the foreseeable future, one question looms large: How soon before they need yet another shot?

Not for many months, and perhaps not for years, according to a flurry of new studies.

Three doses of a Covid vaccine — or even just two — are enough to protect most people from serious illness and death for a long time, the studies suggest.

“We’re starting to see now diminishing returns on the number of additional doses,” said John Wherry, director of the Institute for immunology at the University of Pennsylvania. Although people over 65 or at high risk of illness may benefit from a fourth vaccine dose, it may be unnecessary for most people, he added.

Federal health officials including Dr. Anthony S. Fauci, the Biden administration’s top Covid adviser, have also said that they are unlikely to recommend a fourth dose before the fall.

The Omicron variant can dodge antibodies — immune molecules that prevent the virus from infecting cells — produced after two doses of a Covid vaccine. But a third shot of the mRNA vaccines made by Pfizer-BioNTech or by Moderna prompts the body to make a much wider variety of antibodies, which would be difficult for any variant of the virus to evade, according to the most recent study, posted online on Tuesday.

The diverse repertoire of antibodies produced should be able to protect people from new variants, even those that differ significantly from the original version of the virus, the study suggests.

What’s more, other parts of the immune system can remember and destroy the virus over many months if not years, according to at least four studies published in top-tier journals over the past month.

Specialized immune cells called T cells produced after immunization by four brands of Covid vaccine — Pfizer-BioNTech, Moderna, Johnson & Johnson and Novavax — are about 80 percent as powerful against Omicron as other variants, the research found. Given how different Omicron’s mutations are from previous variants, it’s very likely that T cells would mount a similarly robust attack on any future variant as well, researchers said.

This matches what scientists have found for the SARS coronavirus, which killed nearly 800 people in a 2003 epidemic in Asia. In people exposed to that virus, T cells have lasted more than 17 years. Evidence so far indicates that the immune cells for the new coronavirus — sometimes called memory cells — may also decline very slowly, experts said.

“Memory responses can last for ages,” said Wendy Burgers, an immunologist at the University of Cape Town who led one of the studies, published in the journal Nature. “Potentially, the T-cell response is extremely long lived.”

Throughout the pandemic, a disproportionate amount of research attention has gone to antibodies, the body’s first line of defense against a virus. That’s partly because these molecules are relatively easy to study: They can be measured from a drop of blood.

Analyzing immune cells, by contrast, requires milliliters of blood, skill, specialized equipment — and a lot of time. “It’s orders of magnitude slower and more laborious,” Dr. Burgers said.

Few labs have the wherewithal to study these cells, and their findings lag weeks behind those on antibodies. Perhaps as a result, scientists have frequently overlooked the importance of other parts of the immune system, experts said.