People who haven’t fought off COVID-19 before are still vulnerable to infection from variants after getting the first dose of Pfizer Inc. and BioNTech SE’s vaccine, underscoring the need for fast and full inoculation regimens, according to a U.K. study published Friday.

Among those who previously had mild or asymptomatic cases of COVID, the protection was “significantly enhanced” after a single dose against the variants first seen in the U.K. and South Africa, researchers said in the study, published Friday in the journal Science.

The researchers scrutinized the immune responses -- including antibodies and both T- and B-cells that can fight off viruses -- in U.K. health workers after they received their first doses. Some of them had fought off COVID as far back as early last year.

“Prior infection acts as a prime, which when you boost it with your single-dose vaccine is so effective,” Rosemary Boyton, a professor of immunology and respiratory medicine at Imperial College London and lead author on the study, said in a media call.

The study appears to contrast with other research that has suggested the first dose of Pfizer’s shot provides a strong level of protection against COVID, including some variants. That’s in part because it examined the level of people’s immune responses, rather than real-world infection levels.

Still, the findings offer a warning for countries like the U.K., where the majority of vaccinated people have had only a single dose of inoculations like Pfizer’s that require two shots to offer full protection.

“That’s a potential real vulnerability,” Danny Altmann, another author on the study and a professor of immunology at Imperial College, told reporters.

The solution is for countries to remain cautious in the face of emerging variants, to keep monitoring different strains and to focus on getting people their second doses, Altmann said.