One dose of a coronavirus vaccine may be all that is needed for people who have already been infected with covid-19.
A small study suggests that in people receiving the Pfizer/BioNTech vaccine, the body’s response to natural infection with the coronavirus seems to act like a first dose of the vaccine.
Before the new findings emerged, France had already announced that people there who have been previously infected need only one shot of any two-dose vaccine regimen, the only country to have this policy.
The first three vaccines against the coronavirus to have been approved after full publication of large-scale trials – made by Pfizer/BioNTech, Oxford/AstraZeneca and Moderna – all require two doses given several weeks apart to give the strongest immune response. A single-dose vaccine made by Johnson & Johnson has also been approved in the US and South Africa.
All the vaccines work by mimicking the body’s immune response to a natural infection, usually gauged by measuring the amount of antibodies to a pathogen in the person’s blood. The second shot causes even higher levels of antibodies to be produced.
Mark Mulligan at New York University and his colleagues tracked antibody levels in 32 people who were given both doses of the Pfizer/BioNTech vaccine, and one person who had both doses of the Moderna shot. About half of them had previously been ill with covid-19.
By about two weeks after receiving the first dose, people who had recovered from covid-19 made levels of antibodies that were similar to or higher than those of people who had never been infected – after they had received both doses. This was also the case for their “neutralising antibodies”, ones that are highly effective at defending the body from the virus.
Antibody levels in previously infected people weren’t boosted by their second dose. “These findings support a hypothesis that [previously infected] people may require only a single dose of mRNA vaccine,” Mulligan’s group said in a conference presentation on 9 March. The results were discussed at the Conference on Retroviruses and Opportunistic Infections.
A separate study of 109 people in New York also found similar antibody levels in people who had been infected and had one vaccine dose as in those who had never been infected and had both shots. Participants had received either the Pfizer/BioNTech or the Moderna vaccine. The intensity of systemic side effects such as fever, tiredness and joint pain were also similar in infected people after one dose as in uninfected people after two doses. “People with pre-existing immunity were very worried when they got these side effects because they had not been told to expect them after the first dose,” says Viviana Simon at the Icahn School of Medicine at Mount Sinai, New York, one of the researchers who conducted the study.
“The vaccine is intended to simulate an invading organism. A natural infection will be like in some sense a first dose of the vaccine,” says Stephen Evans at the London School of Hygiene & Tropical Medicine.
As well as antibodies, there are other aspects of the immune response that may still be improved by two doses, such as T-cell activity, says Evans. “If you’re offered two doses, I would take them even if I thought I had been infected, because it would probably boost my overall response and it might make me more likely to respond to a variant,” he says.
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People also can have lower levels of antibodies after a natural infection if they had mild symptoms or none at all, so these people may be more in need of the full two doses, says Stephen Griffin at the University of Leeds, UK.
Last month, France’s High Authority of Health (HAS) said people who had had covid-19, confirmed by a PCR or antigen test, should have one dose of vaccine, between three and six months after their illness, with two exceptions. These are people who have a proven weak immune system, and those who catch covid-19 after their first dose of vaccine. The advice has since been endorsed by the French government. Everyone else is recommended to have two doses, three to four weeks apart.
“Data shows both vaccines currently being used are highly effective after two doses and our programme remains unchanged,” a UK Department of Health and Social Care spokesperson told New Scientist.
Journal reference: NEMJ, DOI: 10.1056/NEJMc2101667