A single dose of the vaccine from Pfizer or Moderna is enough to protect people who have previously been infected with coronavirus, new research suggests.
Two papers, which have not yet been peer-reviewed, found that a single shot of either vaccine produced high concentrations of antibodies, often exceeding the level seen for those given two doses.
People who had previously suffered coronavirus were also more likely to report flu-like side-effects after their vaccine as their immune response was re-stimulated, experts said.
In the first paper, from researchers at the School of Medicine at Mount Sinai New York, 109 people were recruited, of whom 68 were negative for previous infection and 41 had had Covid-19 before.
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The results showed that, after a single dose, people who had been infected previously rapidly developed antibody concentrations which were 10 to 20 times higher than those without prior infection, at the same time points after the jab.
The authors said they are "providing evidence that the antibody response to the first vaccine dose in individuals with pre-existing immunity is equal to or even exceeds" that found in people without prior infection, even after the second dose.
"Changing the policy to give these individuals only one dose of vaccine would not negatively impact on their antibody titers, spare them from unnecessary pain and free up many urgently needed vaccine doses," they wrote.
The second paper, from the University of Maryland in the US, looked at antibody responses to a single dose of the Pfizer/BioNTech or Moderna vaccines in healthcare workers who had had a positive test for Covid-19, and compared them with those who were negative for antibodies to the virus.
Overall, 59 healthcare workers were involved in the study.
The results showed that those who had had Covid-19 previously had statistically significant higher levels of antibodies than those without, and "had a classic secondary response to a single inoculation".
The authors concluded: "In times of vaccine shortage, and until correlates of protection are identified, our findings preliminarily suggest the following strategy as more evidence-based:
"a) a single dose of vaccine for patients already having had laboratory-confirmed Covid-19; and b) patients who have had laboratory-confirmed Covid-19 can be placed lower on the vaccination priority list."
In the UK, health officials have recommended that the second dose of any of the vaccines in use can be safely given up to 12 weeks after the first.
This has proved controversial, with some groups, including the British Medical Association (BMA), calling for doses of the Pfizer jab to be given no more than six weeks apart.
Eleanor Riley, professor of immunology and infectious disease, University of Edinburgh, said of the latest studies: "The data presented in these two papers are not surprising but are very reassuring.
"Both papers show that a previous infection with Covid-19 primes the immune system to make a very robust response to a single dose of Covid-19 vaccine.
"These data indicate that the vaccines are very effectively boosting the immunity induced by infection.
"The authors of both papers suggest that people who have had a PCR-confirmed Covid-19 infection may only require one dose of the vaccine.
"Certainly, this would appear to provide them with protection that is at least as good as two doses of vaccine.
"However, incorporating this into a mass vaccination programme may be logistically complex and it may be safer, overall, to ensure that everyone gets two doses."
She pointed to the evidence suggesting that people previously infected were more likely to experience flu-like symptoms after their vaccine.
"This is good to know and will allow healthcare workers to warn people that they are more likely to feel a little off colour for a day two after their vaccination if they have previously been infected and to reassure them that this is nothing to be concerned about."
Professor Lawrence Young, from the University of Warwick, said: "The antibody levels in previous infected individuals were rapidly elevated in response to a single vaccination reaching 10-20 times those observed in those who were vaccinated without evidence of previous infection.
"Vaccine recipients with pre-existing immunity experienced systemic side-effects with a significantly higher frequency than antibody naive vaccines (fatigue, headache, chills, fever, muscle or joint pains).
"This suggests that the first vaccine dose is serving as a boost in naturally infected individuals, giving levels of protective immunity equivalent to that achieved with two vaccine doses in individuals who haven't been previously infected."
