The Imperial College-led study, which looked at immune responses among healthcare workers in Britain after their first dose of the Pfizer shot, found that those who had previously had mild or asymptomatic infection had enhanced protection against more infectious mutated variants that emerged in Britain and South Africa.
But the immune response after a first dose of the shot was weaker in people who had not previously been infected, potentially leaving them at risk from such variants, researchers leading the work said.
"This study highlights the importance of getting second doses of the vaccine rolled out to protect the population," said Rosemary Boyton, a professor of immunology and respiratory medicine at Imperial who co-led the study.
"People who have had their first dose of vaccine and who have not previously been infected with SARS-CoV-2 are not fully protected against the circulating 'variants of concern'."
The research, published in the journal Science, analyzed blood samples for the presence and levels of immunity against the original strain of SARS-CoV-2 as well as the so-called UK variant, known as B.1.1.7, and another known as B.1.351 that emerged in South Africa.
It found that, after a first dose of the Pfizer vaccine, a boosted immune response in the shape of T-cells, B-cells and neutralizing antibodies was associated with prior infection.
In people who had not had COVID-19, however, a single vaccine dose produced lower levels of neutralizing antibodies against the original virus and its mutant variants.Danny Altmann, an Imperial professor of immunology who co-led the study, said this meant there would be a "window of vulnerability" for people between their first and second doses.