As the fast-spreading coronavirus variant omicron rages through Western Europe, officials and experts in low-vaccinated Eastern Europe anticipate a post-holiday explosion of COVID-19 cases in much of the region.

Many countries in Eastern Europe only recently emerged from infection waves that put a catastrophic strain on health care systems, and at times have tallied some of the highest pandemic death rates globally.

Now, with omicron already confirmed across the region and the winter holidays bringing more community gatherings and international travel, public health officials are predicting a sharp virus surge in the coming weeks.

Adriana Pistol, director of Romania’s National Center for Surveillance and Control of Communicable Diseases, warned Wednesday that the country could see a peak of 25,000 new daily cases during the expected next wave. Romania is the European Union’s second-least vaccinated member nation.

Noting that roughly 60% of Romania’s people over age 65 or living with chronic diseases remain unvaccinated, Pistol said: “Even if the omicron strain does not have the same level of severity(,) ... the health system will be overloaded anyway and reach levels recorded this year in October.”

Romania saw huge lines at borders before Christmas as hundreds of thousands of citizens flocked home, many from the West. The government started requiring travelers to complete passenger locator forms as of Dec. 20 to help track infections, but Pistol said many have failed to fill them out. Only 40% of Romania’s population of around 19 million has been fully inoculated against COVID-19. Although booster doses are considered necessary to provide adequate protection against omicron, Pistol noted that three-quarters of the country’s fully jabbed individuals have yet to receive an additional shot.

Romania’s underfunded public health system teetered on the brink of collapse a few months ago, during the country’s last explosion of virus cases. Hospital morgues ran out of space for bodies, and some patients were transferred abroad for treatment because COVID-19 intensive care units were filled to capacity.

Exhausted medical personnel watched with dread as countries with high vaccination rates such as France, Italy and the U.K. reported record cases as omicron spread in recent days.

“It’s very clear that the fifth wave will probably hit us in January,” Dragos Zaharia, a primary care doctor at the Marius Nasta Institute of Pneumology in Bucharest, said. “We just hope that there will be fewer deaths, fewer severe cases, and fewer hospital admissions.”

Neighboring Bulgaria is the EU’s least-vaccinated member, with just 32% of adults having received a full vaccine. It, too, suffered a deadly fall outbreak, but its vaccine rollout has continued at a sluggish pace. Government data shows that only 255,000 booster jabs have been administered in the country of 7 million people.

“Epidemiologists predict that wave five will hit Bulgaria at the end of January and probably harder in February,” Mariya Sharkova, a public health law specialist based in Plovdiv, told The Associated Press. “Holidays will bring omicron to Bulgaria and probably will have a negative impact on the spread of COVID-19.”

In the Balkans region of Europe, Bosnia, Slovenia, Serbia, and Croatia have all confirmed omicron cases but so far not tightened restrictions to control the variant’s spread.

Serbian authorities decided this week to allow planned outdoor concerts to go ahead on New Year’s Eve. Experts opposed the move and called for requiring COVID-19 status passes for 24 hours due to omicron concerns.

“We are aware that a significant number of our nationals will return home from the European Union for holidays, as well as...foreign guests visiting our ski resorts,” said Goran Cerkez, the assistant health minister in the larger of Bosnia’s two highly autonomous regions. “But we hope that we will weather what is ahead of us.”

The Czech Republic, which has a population of 10.7 million, is among the European countries hardest hit by the pandemic and is currently registering around 6,000 new cases a day. The country’s Health Ministry on Wednesday estimated the omicron variant currently accounts for about 10% of all new cases and by Jan. 10 could be 25%.