The nurse replaced the vaccines between March and April this year at Roffhausen immunization center in Friesland, northwest Germany, the Friesland district administrator, Sven Ambrosy, said on Facebook Tuesday.

"Today I had the sad duty to inform around 8,600 people who may have been affected that it cannot be ruled out they may have received a saline solution instead of their vaccination at their vaccination appointment. For peace of mind we would recommend people get an additional vaccination," Ambrosy said.

The nurse was able to replace the vials because she was "responsible for the preparation of vaccines and the preparation of syringes during her working hours in the vaccination center," health authorities in Lower Saxony said.

Local authorities and the Lower Saxony State Health Office have called on all Germans who received their Covid-19 vaccines at the Roffhausen center between March 5 and April 20, to come forward and get another dose.

"In this situation, it is important that all those who may be affected are offered catch-up vaccinations...This is the only way to ensure complete vaccination protection. Even if persons have already been correctly vaccinated twice," Matthias Pulz, president of the Lower Saxony State Health Office, said in a statement Tuesday.

Police found the nurse involved was "motivated to oppose the vaccination," the Lower Saxony government's coronavirus crisis team, which is overseeing the case, told journalists on Tuesday.

"The investigations of the police have shown the person (involved) was motivated to oppose the vaccination...since she remains silent with police, we do not know whether and to what extent she was manipulated during this period," said Claudia Schröder, the deputy head of the Lower Saxony coronavirus team.

"We do not know many people were specifically affected, and we are talking here about a period of seven weeks, so it is also a large number of people who come into consideration," Schröder added. Saline solution is not considered dangerous for humans.

On Tuesday, German Chancellor Angela Merkel urged Germans to get vaccinated, adding that Germany is seeing a rise in Covid-19 infections due to the Delta variant despite progress on the vaccine rollout.

Merkel said she hopes for "clearly over 70, better 80%" of the German population to get vaccinated. Health Minister Jens Spahn said in a tweet that 55.1% of Germany is fully vaccinated.

From September, elderly and immunocompromised people will be eligible for a third, "booster" shot with an mRNA vaccine, according to the German health ministry.