Eight care home workers in Germany were accidentally injected with five doses of the Covid-19 vaccine, local authorities said Monday -- but are suffering no serious ill effects so far.

The seven women and one man, aged between 38 and 54, are employees of a retirement home in the town of Stralsund in northeastern Germany.

They are therefore in a priority group and among the first to receive the Pfizer-BioNTech Covid-19 vaccine, which Germany began rolling out at the weekend.

But as they were being inoculated on Sunday, they were each injected with five doses at once, according to Stefan Kerth, the administrator of the district where the facility is located.

They were immediately informed of the error, Kerth said.

Four were admitted to hospital for observation with mild flu-like symptoms, while the others were sent home.

-I deeply regret this incident- Kerth said, adding that the mistake was an isolated case and down to human error.

-I hope all those affected suffer no serious side effects- he said.

Ahead of the launch of vaccination campaigns in several European countries at the weekend, German company BioNTech said larger than normal doses had been injected during trials without serious side effects.

The vaccine, created in less than a year and designed to be administered in two injections, is being delivered in vials that each contain five doses once diluted.

However, a spokeswoman for the health ministry said Monday that the vials could each be used to give up to six doses.