Almost 30 million people in the United States – and likely many others whose diseases have never been diagnosed – have been infected with the coronavirus to date. Should these people still be vaccinated?

Two new studies answer this question with an emphatic yes.

In fact, research suggests that for these people, just one dose of the vaccine is enough to charge their antibodies and destroy the coronavirus – and even some other infectious variants.

The results of these new studies are consistent with the results of two others published in the past few 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 really strong rationale for 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 varies widely. Most people make plenty of antibodies that last for many months. However, some people who have had mild or no symptoms of Covid-19 produce few antibodies that quickly drop to undetectable levels.

The vaccines “even hit the pitch,” said Dr. Gommerman, so that anyone who has recovered from Covid-19 will make enough antibodies to protect against the virus.

The latest study, which has not yet been published in a scientific journal, analyzed blood samples from people with Covid-19. The results suggest that her immune system would have problems fighting off B.1.351, the coronavirus variant first identified in South Africa.

But a shot of the Pfizer BioNTech or Moderna vaccine changed the picture dramatically: It increased the amount of antibodies in her blood by a thousand times – “a massive, massive surge,” said Andrew T. McGuire, immunologist at Fred Hutchinson Cancer Research Center in Seattle, who led the study.

Rinsed with antibodies, samples from all participants were able to neutralize not only B.1.351, but also the coronavirus that caused the SARS epidemic in 2003.

In fact, the antibodies appeared to work better than those in people who did not have Covid and had received two doses of a vaccine. Several studies have shown that the Pfizer-BioNTech and Moderna vaccines are about five times less effective against the variant.

The researchers received blood samples from 10 volunteers in the Seattle Covid Cohort Study who were vaccinated months after contracting the coronavirus. Seven of the participants received the Pfizer BioNTech vaccine and three received the Moderna vaccine.

Blood taken about two to three weeks after vaccination showed a significant increase in antibody levels compared to the samples taken 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.

Updated

Apr. 19, 2021 at 1:19 am ET

The researchers also saw a surge in immune cells remembering and fighting the virus, said Dr. McGuire. “It looks pretty clear that we are boosting their pre-existing immunity,” he said.

In another new study, New York University researchers found that a second dose of the vaccine was of no great benefit at all for people with Covid-19 – a phenomenon that has also been seen with vaccines against other viruses.

In this study, most people had been infected with the coronavirus eight or nine months previously, but their antibodies increased hundreds to a thousand times with the first dose of a vaccine. However, after the second dose, the antibody levels did not rise any further.

“It is real evidence of the strength of immunological memory that they are given a single dose and have a huge increase,” said Dr. Mark J. Mulligan, director of the NYU Langone Vaccine Center and lead author of the study.

In some parts of the world, including the United States, a significant minority of the population is already infected, noted Dr. Mulligan firmly. “You should definitely be vaccinated,” he said.

It is unclear whether the thousand-fold increase in antibody levels recorded in the laboratory will occur in real-world environments. However, research shows that a single shot is enough to significantly raise antibody levels, said Florian Krammer, an immunologist at the Icahn School of Medicine on Mount Sinai in New York.

Dr. Krammer led another of the new studies that showed that people who had Covid-19 and received a dose of vaccine had more serious side effects from the vaccination and had more antibodies than those who had not been infected before.

“When you put all four papers together, you get pretty good information about people who have had an infection and only need one vaccination,” said Dr. Krammer.

He and other researchers are trying to convince scientists from the Centers for Disease Control and Prevention to recommend only one dose for those who have recovered from Covid-19.

Ideally, these people should be monitored after the first shot in case their antibody levels drop after a few weeks or months, said Dennis R. Burton, an immunologist at the Scripps Research Institute in La Jolla, California.

The fact that the charged antibodies seen in the new study can fight the 2003 SARS virus suggests that a single dose of the vaccine may have induced the volunteer’s bodies to produce “largely neutralizing antibodies” – immune molecules that are able to target a wide range of related antibodies to viruses, said Dr. Burton.

He and other scientists have spent decades investigating whether largely neutralizing antibodies can fight multiple versions of HIV at the same time. HIV mutates faster than any other virus and evades most antibodies quickly.

The new coronavirus is mutating much more slowly, but there are now several variants of the virus that appear to have become more contagious or which are thwarting the immune system. The new study could provide clues on how to make a single vaccine that stimulates the production of largely neutralizing antibodies that can destroy all variants of the coronavirus, said Dr. Burton.

Without such a vaccine, scientists would have to adjust the vaccines every time the virus changes significantly. “You’re kind of a whac-a-mole approach,” he said. It will likely take many months, if not more, to develop and test this type of vaccine against the coronavirus, but “this is the longer term way to approach this virus.”