The first study, which was published as a preprint and has not yet been peer reviewed, found that seropositive individuals had a rapid antibody response after one dose of either the Pfizer-BioNTech or Moderna vaccines. 1 It also found that people who had previously had covid-19 reported side effects more frequently after the first dose—in line with the level reported for the second dose in phase III vaccine trials.

The researchers, from Mount Sinai in New York and Paris, examined the antibody responses in 109 people—68 of them had never had covid-19, whereas 41 had previously tested positive. The antibody titres of those who had previously had covid-19 were 10-20 times those of people who had never had it at the same time points. Two weeks after a single dose of vaccine, people who had previously had covid-19 had antibody concentrations that were as high, or up to 10 times higher, than the levels seen in uninfected people who had received two doses of the vaccine.