Around 50 people vaccinated in the Pirkanmaa region will need to be revaccinated as they were given doses that were too small.
Using the so-called 'air bubble technique', shown at official vaccine training sessions, it is possible for vaccinators to inadvertently underdose when using the Pfizer and Biontech vaccine.
At the end of last year, Finland changed its guidance on vaccination after it emerged that healthcare workers may have thrown out hundreds of usable doses of Pfizer’s Covid-19 vaccine. The new guidance stated that healthcare providers can use all the available vaccine in a vial.
A mistake can happen if a so-called tuberculin syringe is used, as the vaccine dosage cannot be reliably estimated.
The official number of doses in a vaccine vial is six doses, but with the air bubble technique, seven or even eight doses can be extracted from a vial.
The Finnish institute for Health and Welfare (THL) has approved the use of extracting extra vaccine doses from vials, but stresses that all doses must be full.
The Pirkanmaa Hospital District stated that the problem was detected in the town of Nokia.
"This came to light yesterday. We were able to determine that the incorrect doses were administered only on 24 February," says Katja Ylitolva, medical director at the town of Nokia.
The Pirkanmaa hospital district immediately notified THL when they became aware of the issue. THL has since updated its vaccination guidance to avoid underdosing.
To date, more than 30,000 people have been vaccinated in Pirkanmaa.
As of 25 February, 319,559 people (5.8% of Finland's population) had received at least one vaccination dose.