A longer interval between the first two doses of the COVID-19 vaccine may boost antibody levels up to nine-fold, according to a study conducted in the UK.

The finding by researchers at the UK Health Security Agency (UKHSA) also suggests that eight months after primary infection may be an optimum time to receive the first vaccine in those with prior infection. However, the analysis shows that regardless of the timing between infection and vaccination, all individuals mount a very high antibody response after the second dose.

In the yet-to-be-published study, the researchers measured antibody levels in blood samples taken from almost 6,000 healthcare workers from across the UK. As many as 3,989 of the 5,871 study participants had their first dose of the Pfizer/BioNTech COVID-19 vaccine at least 21 days earlier, while 1,882 had their second dose at least 14 days earlier.

The participants were classified by infection history as either previously having had COVID-19 -- which was confirmed by a PCR test or assumed due to their antibody profile -- or naive, with no history of infection. Almost all (over 99 per cent) of those who had not had COVID-19 after vaccination developed antibodies against the SARS-CoV-2 virus, the researchers said.

After the first dose, those with the previous infection had up to ten times higher antibody levels than naive individuals, while after the second dose, those with the previous infection had antibody levels more than twice as high as those who had not had a previous infection, they said.

When analysing dosing intervals, the researchers found that longer dosing interval was associated with antibody levels that were up to nine times higher in naive participants with a more pronounced effect observed in younger participants.

"This study shows that a long time between vaccine dose 1 and dose 2 results in higher antibody responses in naive participants, which strongly supports the decision by the UK government to lengthen the interval between vaccine doses," said Ashley Otter, from UK Health Security Agency.

"We have also shown that in those with the previous infection, the 'timing between exposure and vaccination plays a critical role in post-vaccination antibody responses," Otter said in a statement. However, the researchers said further research is needed to determine whether these higher antibody levels provide greater protection against COVID-19 disease, and how this longer dosing interval may affect booster responses.

The research is being presented at this year's European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Lisbon, Portugal, held between April 23-and 26. The dosing interval did not affect antibody levels in those previously infected. However, a longer interval between infection and vaccination was linked to higher antibody levels, the researchers said.

Those who had their first dose of the vaccine eight months after an infection had antibody levels seven times higher than those who were vaccinated three months after infection, with a plateau after eight months, they said

The study also found that female participants and those from ethnic minorities were associated with significantly higher antibody levels, while immunosuppression was associated with significantly lower post-vaccination antibody responses.