Five European nations have recently been added to the U.S. 'do not travel' list just weeks ahead of the holiday season.

As the busy December holiday season approaches, the U.S. Centers for Disease Control and Prevention has raised its advisory level for several popular European countries.

On Monday, Dec. 6, the CDC classified Portugal, Liechtenstein, France, Cyprus, and Andorra under its highest warning, "CDC Level 4: COVID-19 Very High."

The CDC employs a four-tiered system to classify international destinations, with level four indicating a "very high" COVID-19 risk. U.S. travelers are strongly advised to avoid these areas.

This classification is likely linked to the rise of the omicron coronavirus variant, which the World Health Organization has labeled a "variant of concern." According to the WHO, omicron may be more contagious and less susceptible to COVID-19 vaccines compared to previous variants. So far, it has been detected across six continents and in 19 U.S. states, as reported by The New York Times.

Late last month, the Biden Administration suspended entry into the U.S. for travelers from eight South African countries, including South Africa, Botswana, and Zimbabwe. This temporary ban affects anyone who has been in these nations within the 14 days prior to their attempted entry into the U.S., but it does not apply to American citizens or permanent residents.

The White House also implemented new testing requirements for all passengers on flights heading to the U.S., reducing the testing window from three days to just one day.

However, the CDC's classification could dampen holiday travel, which is expected to benefit the struggling international travel sector. For example, Air France increased its service between France and the U.S. this winter, anticipating higher demand for travel between the two countries.

Additionally, countries have signaled their willingness to tighten restrictions if needed. Portugal reported nearly 25,000 new coronavirus cases in the past week, along with 121 deaths. Given the surge in cases, the country stated it would not hesitate to impose new measures, even during the winter holiday season. The government has already reinstated indoor mask mandates for everyone.

"We all hope these measures won't be necessary, but if they are, we will enforce them," said Portugal's Prime Minister Antonio Costa last week.

So far, it appears that travelers are not significantly altering their booking behaviors due to concerns over the omicron variant. A study conducted by the travel deals site Scott's Cheap Flights revealed that nearly 90% of travelers have no plans to cancel their holiday travel arrangements.