Dr. Hans Kluge described a "west-to-east tidal wave" of Omicron, accompanied by a surge in the Delta version.
The estimate was based on 7 million new cases in Europe in the first week of 2022.
Infections have doubled in two weeks.
"Today the Omicron variation reflects a fresh west-to-east tidal wave, sweeping through the region on top of the Delta surge," Dr. Kluge said.
The Institute for Health Metrics and Evaluation in Seattle predicts "about 50% of the region's population will be infected with Omicron in the next six to eight weeks."
The virus had moved from western countries into the Balkans, he claimed.
A country's response must take into account its epidemiological, resource, vaccine, and socio-economic contexts.
Research shows Omicron is less likely to cause significant illness than prior Covid versions. Omicron is still very contagious and can infect even completely immunised people.
The unprecedented number of cases has put strain on health systems.
On Monday, the UK recorded 142,224 new cases and 77 deaths. Several hospitals have declared "critical" occurrences due to staff absences and Covid-induced pressure.
Hospitalization rates are also growing. Last week, France's Health Minister Olivier Veran warned of a difficult January.
He said that Omicron patients occupied "normal" hospital beds while Delta patients strained ICUs.
Poland reported 100,000 virus-related deaths since the outbreak began. Poland presently has the world's sixth-highest Covid-19 death rate, and nearly 40% of its population is unvaccinated.
Anna Popova, the Russian consumer health minister, told a coronavirus task force conference that 100,000 new Covid cases may be reported daily if no action was taken.
The daily infection rate had lately fallen from a high of 41,335 instances in early November, according to Reuters.
Ms Popova said 305 Omicron instances had been identified in 13 of the country's regions. To date, Russia has recorded 311,281 deaths and 10.5 million cases.
Pfizer announced on Monday that it will be able to sell a vaccine that protects against Omicron in March. Experts believe it's unclear if this is required.