One dose of Pfizer offers strong protection for those who have had Covid-19, according to studies.

For people who have had Covid-19, a single dose of the Pfizer vaccine is enough to provide robust protection against the coronavirus, according to two new studies from Britain published Thursday night in The Lancet, a leading journal. medical.

The studies, among the first articles fully vetted to assess how to vaccinate people who have had Covid-19, added strong evidence to the case of inoculating people who already have antibodies to the virus, but only with one dose of the Pfizer vaccine. . .

One of the studies, led by researchers from University College London and Public Health England, described the benefits of that strategy.

“This could speed up the launch of the vaccine,” they said. And that, in turn, could prevent dangerous new mutations: “Wider coverage without compromising vaccine-induced immunity could help reduce the occurrence of variants,” the paper said.

In recent weeks, several studies on the subject that have yet to be published in scientific journals have been published online, showing that a dose of a coronavirus vaccine amplifies people’s antibodies from a previous infection.

People’s immune responses to infection are highly variable: Most people produce substantial and long-lasting antibodies, while others with milder infections produce relatively few, making it difficult to know how protected they are from the virus.

Vaccines act as a kind of booster for these people’s immune responses, inducing enough antibodies to offer protection. But a single dose, rather than the full two-dose protocol, is sufficient for those who have been infected, several studies have suggested.

Some researchers in the United States are trying to persuade the Centers for Disease Control and Prevention to recommend giving only one dose to people who have recovered from Covid-19. The British studies are likely to put pressure on health officials to consider the same approach.

More than 28 million people in the United States and four million people in Britain, along with many others whose diseases were probably never diagnosed, have been infected so far.

One of the new studies, led by Charlotte Manisty, a professor at University College London, and Ashley D. Otter, a research scientist at Public Health England, tracked 51 healthcare workers in London who have undergone routine tests for antibodies and infections since March. . That gave the researchers an unusually detailed picture of any pre-existing protection against the virus.

About half of the healthcare workers had experienced a mild or asymptomatic infection. And a single dose of the Pfizer vaccine increased their antibody levels more than 140 times from their peak levels before being inoculated, according to the study. That appeared to give them better protection against the coronavirus than two doses of the vaccine in people who had never been infected, the researchers wrote.

The study floated the idea of ​​testing people’s blood in the weeks before they were eligible for a Pfizer vaccine to determine if they already had antibodies. People’s immune responses to infection are highly variable, making it difficult to predict without a blood test who can be fully protected with a single dose.

As an added benefit of the single-dose strategy, the researchers wrote that it would spare people who have already been infected the unpleasant side effects that sometimes follow a booster injection in that group.

The second study, led by scientists from Imperial College London, measured the immune responses of 72 healthcare workers who were vaccinated in late December. A third showed signs of having been previously infected.

For those people, a dose of the Pfizer vaccine stimulated “very strong” antibody responses, according to the study, as well as “very strong T-cell responses,” referring to another arm of the immune system.

It is not clear how long the post-vaccine immune response will last in people who have already been infected compared to those who have not.

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