BRUSSELS — After struggling with the coronavirus for far too long, the world understands all too well Belgium’s word of the year, “knaldrang!” — the urge to party, the need to let loose.

Yet as New Year celebrations approach, the omicron variant is casting more gloom.

Dire warnings abound, caseloads are rising alarmingly fast, air traffic is snarled and several countries are considering more restrictions to add to the patchwork of lockdowns and other measures already in place around Europe. The top U.S. infectious-disease expert, Dr. Anthony Fauci, warned on Monday that with the rise of the highly contagious omicron, “it’s going to get worse before it gets better.”

New York City’s sweeping mandate requiring nearly all businesses, from multinational corporations to corner grocery stores, to ban unvaccinated employees from the workplace took effect Monday.

In Denmark, infection numbers have gone up drastically in the past few days and set a one-day record of over 16,000 in the nation of 5.8 million.

Travelers around the world faced flight cancellations and delays because of staffing shortages linked to a flight-tracking website, counted more than 2,400 cancellations worldwide by Monday afternoon — 884 of them within, into or out of the U.S.