Thousands in Germany get extra shots after a nurse is suspected of tampering with the vaccine.

After a nurse admitted to replacing the contents of a Covid vaccine vial with a saline solution in a small German inoculation center in April, the local authorities said on Tuesday that she might have systematically substituted vaccines for saline solution over a period of weeks while working at the center.

The authorities are now asking all 8,557 people who were jabbed during the nurse’s working hours to return for third, and in some rare cases fourth, jabs as a precautionary measure. She worked at the center, near the town of Wilhelmshaven on the North Sea coast, for seven weeks in March and April.

Although the saline solution itself is harmless, anyone injected with it would have missed out on the protection of a Covid vaccine.

“It is quite deceitful to sneak into a vaccination center with the intention to do something like that,” Heiger Scholz, who leads the Lower Saxony coronavirus task force, said at a news conference on Tuesday.

He said there were indications that the nurse might be against vaccines.

A police investigation found that the nurse had shared social media posts criticizing the government’s pandemic restrictions.

The criminal investigation is focusing on accusations that the nurse’s switching of the initial vial resulted in saline solution being injected instead of six vaccine doses. But the police now say there is a credible chance that the nurse intentionally substituted vaccines on other occasions while working in the laboratory of the Friesland vaccination center. She was responsible for preparing vaccine syringes.

Officials have not said whether they plan to charge her with any crime. A lawyer representing the nurse told the German news service DPA that she had switched just one vial.

The nurse, who has not been charged with any offense, admitted in April that she had refilled a broken Pfizer-BioNTech vaccine vial with a saline solution, in an episode that she described as an accident.

Unable to pinpoint exactly which vials had been compromised in April, the authorities invited 117 people to take antibody tests. They also offered additional vaccine shots to another 80 who had been to the center for second doses.