Authorities in northern Germany appealed to thousands of people on Tuesday to get another shot of COVID-19 vaccine after a police investigation found that a Red Cross nurse may have injected them with a saline solution.

The nurse is suspected of injecting salt solution into people's arms instead of genuine doses at a vaccination centre in Friesland - a rural district near the North Sea coast - in the early spring.

"I am totally shocked by this episode," Sven Ambrosy, a local councillor, said on Facebook as local authorities issued the call to around 8,600 residents who may have been affected.

While saline solution is harmless, most people who got vaccinated in Germany in March and April - when the suspected switch took place - are elderly people at high risk of catching the potentially fatal viral disease.

Police investigator Peter Beer, speaking earlier at a news conference covered by German media, said that based on witness statements there was "a reasonable suspicion of danger".

The motive of the nurse, who was not named, was not clear but she had aired sceptical views about vaccines in social media posts, police investigators said.

It was not immediately clear whether the suspect had been arrested or charged in the case, which according to broadcaster NDR has been handed to a special unit that investigates politically motivated crimes.

Local police declined to comment outside of normal working hours.