Eight healthcare workers in Germany were accidentally injected with five times the recommended dose of the BioNTech-Pfizer vaccine, Reuters reports.
Four of the workers at the Stralsund city elderly care home in the Vorpommern-Ruegen district have gone to hospital for observation after developing flu-like symptoms in response to the overdose.
"I deeply regret the incident," said district chief Stefan Kerth, the administrator of the area where the elderly care home is located.
"This individual case is due to individual errors. I hope that all those affected do not experience any serious side-effects."
The mishap comes as Germany begins its vaccine rollout.
The country had a relatively low death rate in the first phase of the pandemic but has been hard-hit by a second wave of the virus.
Chancellor Angela Merkel earlier this month instituted a "hard lockdown" to try to curb the spread of Covid-19.
The new restrictions meant shops and schools were closed until January. Social gatherings are also limited throughout the Christmas period.
Germany's confirmed death toll in the coronavirus pandemic has topped 30,000. Among major European nations, Italy, the UK, France and Spain still have higher death tallies.
- additional reporting: AP