A single dose of one of the two-shot vaccines for the coronavirus disease 2019 (Covid-19) prevented an estimated 95 percent of new infections among healthcare workers two weeks after receiving the jab, a study published Wednesday by JAMA Network Open found.

The first dose of the Moderna vaccine was 78-percent effective at preventing new cases just one week after clinical staff were inoculated, the data showed.

Still, 39, or just over 1 percent of the nearly 3,400 health workers who received the vaccine later became infected with the virus, said researchers, from the VA Boston Healthcare System.

These “breakthrough” infections occurred at least 14 days after they had received their first dose, and 27 of them developed symptoms of the disease.

“[Even] at the height of Boston’s winter surge, … rapid first-dose vaccination of healthcare workers … helped sustain healthcare delivery,” study co-author Dr. Michael E. Charness said.

However, “full vaccination is still recommended for sustained immunity and protection against emerging variants,” said Charness, chief of staff at the VA Boston Healthcare System.

Both two-dose Covd-19 vaccines, from Pfizer-BioNTech and Moderna, offer more than 90% protection against serious illness from the virus, research found.

They work by using modified genetic material from the coronavirus to stimulate the production of antibodies, or cells that fight off infections, by the immune system, according to the Centers for Disease Control and Prevention.

This essentially primes the immune system to respond when or if a vaccinated person is exposed to the virus, the agency says.

Although studies have found that one dose of either of the two-shot vaccines offers at least some protection against the virus, they are most effective among those who are considered “fully vaccinated,” defined as two weeks after receipt of the second shot.

For this study, Charness and his colleagues assessed the effectiveness of one dose of the Moderna vaccine in more than 4,000 healthcare workers at the VA Boston Healthcare System during the winter spike of Covid-19 cases in the Boston area.

Just under 3,400, or about 84 percent, of the staff had received one dose of the vaccine by the end of the 42-day study period on Feb. 1, the researchers said.

Among the included healthcare workers, 107 new Covid-19 cases were reported during the study period, the data showed.

Of these, 39 were in those who had received their first dose and 68 occurred in those who had not received a shot, according to the researchers.

“Vaccine effectiveness may vary depending on the age, exposure and health risks of a population and transmissibility of variants,” Charness said.

“Our work was done at the time the [original Wuhan variant was dominant, [and] efficacy might be different with the currently prevalent variants,” such as the B.1.1.7 or Delta strains, he said.