So far, nearly 30 million people in the United States – and probably many others who have never been diagnosed with the disease – have been infected with the coronavirus. Do these people still need to be vaccinated?
Two new studies answer that question with an emphatic yes.
In fact, the research suggests that just one dose of the vaccine is enough for these people to load up their antibodies and destroy the coronavirus – and even some more contagious variants.
The results of these new studies are consistent with the findings of two others published in recent weeks. Taken together, the research suggests that people who have had Covid-19 should be immunized – but a single dose of the vaccine may be enough.
"I think it's a very strong reason why people who were previously infected with Covid should get the vaccine," said Jennifer Gommerman, an immunologist at the University of Toronto who was not involved in the new research.
A person's immune response to a natural infection is highly variable. Most people make large amounts of antibodies that last for many months. But some people who had mild symptoms or no symptoms of Covid-19 produce few antibodies, which quickly drop to undetectable levels.
The vaccines "even hit the playing field," said Dr. Gommerman, so that anyone who has recovered from Covid-19 produces enough antibodies to protect themselves against the virus.
The latest study, which has not yet been published in a scientific journal, analyzed blood samples from people who have had Covid-19. The findings suggested that their immune systems might have difficulty fighting off B.1.351, the coronavirus variant first identified in South Africa.
But one shot of the Pfizer-BioNTech or Moderna vaccine changed the picture significantly: It increased the amount of antibodies in their blood a thousand-fold – "a huge, huge boost," said Andrew T. McGuire, an immunologist at Fred Hutchinson. . Center for Cancer Research in Seattle, who led the study.
Rinse with antibodies, samples from all participants were able to neutralize not only B.1.351, but also the coronavirus that triggered the SARS epidemic in 2003.
In fact, the antibodies seemed to perform better than those in people who had not had Covid and received two doses of a vaccine. Multiple studies have suggested that the Pfizer-BioNTech and Moderna vaccines are about five times less effective against the variant.
The researchers obtained blood samples from 10 volunteers in the Seattle Covid Cohort Study who were vaccinated months after contracting the corona virus. Seven of the participants received the Pfizer-BioNTech vaccine and three received the Moderna vaccine.
Blood collected approximately two to three weeks after vaccination showed a significant increase in antibody levels compared to the samples collected before vaccination. The researchers don't yet know how long the increased levels of antibodies will last, but "hopefully they will last," said Dr. McGuire.
The researchers also saw an increase in immune cells that remember and fight the virus, said Dr. McGuire. "It looks pretty obvious that we are strengthening their pre-existing immunity," he said.
In another new studyResearchers at New York University found that a second dose of the vaccine did not provide much benefit at all to people who have had Covid-19 – a phenomenon that has also been seen with vaccines for other viruses.
In that study, most people had been infected with the coronavirus eight or nine months earlier, but saw their antibodies increase a hundred-fold to a thousand-fold when given the first dose of a vaccine. However, antibody levels did not increase further after the second dose.
"It's a real testament to the power of the immunological memory that they get a single dose and have a massive increase," said Dr. Mark J. Mulligan, director of the N.Y.U. Langone Vaccine Center and the lead author of the study.
In some parts of the world, including the United States, a significant minority of the population is already infected, Dr. Mulligan. "They definitely need to be vaccinated," he said.
It is unclear whether the 1,000-fold peak in antibody levels recorded in the laboratory will occur in practice. Still, the research shows that a single injection is enough to significantly boost antibody levels, said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York.
Dr. Krammer led another one of the new studies, which showed that people who have had Covid-19 and received one dose of a vaccine experienced more serious side effects from the vaccination and had more antibodies compared to those who had not been previously infected.
"If you put all four documents together, it provides some pretty good information about people who have already had an infection who only need one vaccination," said Dr. Krammer.
He and other researchers are trying to convince scientists at the Centers for Disease Control and Prevention to recommend just one dose for those who have recovered from Covid-19.
Ideally, those people should be monitored after the first injection in case their antibody levels drop after several weeks or months, said Dennis R. Burton, an immunologist at the Scripps Research Institute in La Jolla, California.
The fact that the supercharged antibodies seen in the new study can fight the 2003 SARS virus suggests that a single dose of the vaccine may have prompted the bodies of the volunteers to produce "broadly neutralizing antibodies". – immune molecules capable of producing a wide variety of related viruses, said Dr. Burton.
He and other scientists have spent decades investigating whether broadly neutralizing antibodies have multiple versions of H.I.V. can tackle. instantly. H.I.V. mutates faster than any other virus and evades most antibodies quickly.
The new coronavirus mutates much more slowly, but there are now multiple variants of the virus that appear to have evolved to be more contagious or thwart the immune system. The new study may provide clues as to how create a single vaccine that stimulates the production of commonly neutralizing antibodies that can destroy all variants of the coronavirus, said Dr. Burton.
Without such a vaccine, scientists will have to adjust the vaccines every time the virus changes significantly. “You're stuck in some kind of Whac-a-Mole approach,” he said. It will likely take many months, if not longer, to develop and test such a coronavirus vaccine, but "that is the long-term way to approach this virus."