Single dose of Pfizer vaccine offers robust protection for those who have had Covid-19, studies find.

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

The studies, among the first fully vetted papers to weigh in on how to vaccinate people who have had Covid-19, added strong evidence to the case for giving just one dose of the Pfizer vaccine to people who already have antibodies against the virus.

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

“This could potentially accelerate vaccine rollout,” they said. And that in turn could forestall dangerous new mutations: “Wider coverage without compromising vaccine-induced immunity could help reduce variant emergence,” the paper said.

In recent weeks, several studies on the topic were posted online but not yet published in scientific journals, fueling discussions over strategies for vaccinating people who were previously sick. One dose was also enough in those studies to amplify people’s antibodies from an earlier infection.

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 studies from Britain seem likely to put pressure on health officials there 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 illnesses 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 health workers in London who have submitted to routine tests for antibodies and infection since March. That gave researchers an unusually detailed picture of any pre-existing protection from the virus.

Roughly half of the health workers had been infected, and a single dose of the Pfizer vaccine increased their antibody levels more than 140-fold from their peak levels before being inoculated, the study said. That appeared to give them better protection against the coronavirus than two doses of the vaccine did in people who had never been infected, the researchers wrote.

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

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

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

For those people, one dose of the Pfizer vaccine stimulated “very strong” antibody responses, the study said, 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 previously been infected compared with those who have not.

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