An ICU hospital Christmas party that occurred near Madrid, is suspected to be the cause of a massive outbreak of COVID-19 cases among some of its staff workers.

Prime Minister Pedro Sánchez told reporters on Monday as a response to the news, that people should “remain prudent” of COVID over the holidays, and “we must not lower our guard.”

A total of 68 nurses and doctors from University Regional Hospital in Malaga tested positive and were diagnosed with coronavirus, the Andalusian regional government said according to Reuters.

All of the infected staff members attended the Christmas party, which Health authorities see as the most viable reason for the spread. The party occurred at a restaurant on Dec. 1 and saw 173 staff members in attendance.

To attend the party, staff had to show proof of a negative antigen test before the event, and all members were already vaccinated. This has led to some authorities questioning how such a large outbreak could have been capable of happening, according to Spanish private news agency Europa Press.

Antigen tests are faster and less expensive but also less accurate than other Covid tests like PCR tests.

The infected staff members are now isolating.

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