Antibody levels against SARS-CoV-2, the virus that causes COVID-19, stay more durable in people who were infected by the disease and then received two doses of vaccine compared with those who only got immunised, according to a study.

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The research, published in the Journal of the American Medical Association on 1 November, followed 1,960 health care workers in the US who had received both doses of either the Pfizer/BioNTech or Moderna vaccines. This included 73 people who had a positive SARS-CoV-2 polymerase chain reaction (PCR) test result before the first vaccine dose.

The 73 were divided into two groups those who were infected at 90 days or closer to the first vaccine dose, and those whose exposure to the virus was more than 90 days before the initial shot. After adjusting for vaccine type, age and sex, antibody levels were compared for those with and without prior SARS-CoV-2 infection at one, three and six months following the second vaccine dose.

The antibody levels were also compared at one and three months following the second dose between the two groups with prior SARS-CoV-2 infection. The two mRNA vaccines evaluated in the study introduce the body's immune system to S1, a protein subunit that is a component of the spikes found on the surface of SARS-CoV-2.

The spikes enable the virus to latch onto healthy cells and infect them. Immunoglobulin G antibodies, elicited by S1 from the vaccines stimulate the immune system, neutralise the virus particles, prevent infection by SARS-CoV-2, or at least reduce the severity of the disease.

"We found that health care workers with prior SARS-CoV-2 infection followed by two doses of mRNA vaccine, therefore, three independent exposures to the S1 spike protein -- developed higher antibody levels than those with vaccination alone," said study lead author Diana Zhong, a fellow at the Johns Hopkins University School of Medicine, US.

"The relative differences were 14 per cent higher at 1 month following the second vaccine dose, 19 per cent at three months and 56 per cent at six months," Zhong said. The study participants with a PCR-confirmed SARS-CoV-2 infection more than 90 days before their initial vaccination had adjusted antibody levels 9 per cent and 13 per cent higher than those who were exposed to the virus less than or equal to the 90-day mark.

"This suggests that a longer interval between infection and first vaccine dose may enhance the antibody response," said study senior author Aaron Milstone, a professor at the Johns Hopkins University School of Medicine.

Further investigation is needed to determine whether increased post-vaccination durability in previously infected people is attributable to the number of exposures to the virus, the interval between exposures, or the interplay between natural or vaccine-derived immunity, Milstone added.