For people taking Covid-19, a single dose of the Pfizer vaccine is sufficient to provide robust protection against the coronavirus, according to two new studies from Britain published on Thursday in The Lancet, a leading medical journal.
The studies, among the first fully controlled articles to assess how to vaccinate people who took Covid-19, added strong evidence to the case of inoculating people who already have antibodies to the virus – but only with a dose of the Pfizer vaccine.
One of the studies, led by researchers at University College London and Public Health England, described the benefits of this strategy.
“This could speed up the launch of the vaccine,” they said. And that, in turn, can prevent new dangerous mutations: “Broader coverage without compromising vaccine-induced immunity can help to reduce the appearance of variants,” the newspaper said.
In recent weeks, several studies on the subject have been posted online that have not yet been published in scientific journals, showing that a dose of a vaccine against the coronavirus has amplified the antibodies of a previous infection.
The immune responses of people when they are infected are highly variable: most people produce considerable and long-lasting antibodies, while others who have had 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 the immune responses of these people, 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. Britain’s studies appear to pressure local health officials to consider the same approach.
More than 28 million people in the United States and four million in Britain, along with many others whose diseases have probably never been 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 – screened 51 health professionals in London who have undergone routine tests for antibodies and infection since March. This gave researchers an exceptionally detailed picture of any pre-existing protection against the virus.
Approximately half of the healthcare professionals had a mild or asymptomatic infection. And a single dose of the Pfizer vaccine increased their antibody levels by more than 140 times the maximum levels before being inoculated, the study said. This seemed 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 raised the idea of having blood tests in the weeks before they became 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 save people who have already been infected from the unpleasant side effects that sometimes follow a booster injection in that group.
The second study, led by scientists at Imperial College London, measured the immune responses of 72 health professionals who were vaccinated in late December. A third showed signs of having been previously infected.
For these people, a 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 unclear how long the post-vaccine immune response will last in people who have already been infected compared to those who have not.