The head of the U.N. health agency said Wednesday holiday gatherings and the spread of the most prominent variant globally led to increased transmission of COVID-19 last month.

Tedros Adhanom Ghebreyesus said nearly 10,000 deaths were reported in December, while hospital admissions during the month jumped 42% in nearly 50 countries — mostly in Europe and the Americas — that shared such trend information.

“Although 10,000 deaths a month is far less than the peak of the pandemic, this level of preventable deaths is not acceptable,” the World Health Organization director-general told reporters from its headquarters in Geneva.

He said it was “certain” that cases were on the rise in other places that haven’t been reporting, calling on governments to keep up surveillance and provide continued access to treatments and vaccines.