ZURICH -- Austria should offer some healthcare staff and other key workers a fourth COVID-19 jab, the National Vaccination Board said in updated guidance published late on Thursday, citing risks from the fast-spreading Omicron variant of the coronavirus.

But it said there was not enough scientific data for it to recommend fourth shots be given more widely.

"In view of an imminent Omicron wave, (a fourth vaccination) can be offered in high-risk areas (e.g., exposed health care personnel) and in systemically critical areas from six months after the third vaccination," the board said.

Austria said on Wednesday it will tighten restrictions again from Dec. 27 to slow the spread of the Omicron variant.

In November, the government announced a fourth national lockdown and said it would make vaccinations compulsory for all Austrians, the first European Union country to do so.

While that three-week lockdown slashed daily COVID-19 infections from record levels, the country is bracing for another surge. Several hundred Omicron cases have been confirmed already.

In its updated guidelines, the National Vaccination Board said fourth COVID-19 shots could be given "off label" to groups in healthcare and other critical areas following medical consultation.

"There is still no evidence that this additional vaccination can prevent infections. However, it can be it can be assumed that serious illnesses can be prevented," it said.