The Centers for Disease Control and Prevention (CDC) has advised Americans to avoid traveling by cruise ships regardless of their vaccination status amid a global surge of coronavirus cases.

In a statement released Thursday, the health agency’s COVID-19 Travel Health Notice updated the warning from level 3 to level 4, which is the health agency’s highest level of warning.

The agency said even people who are vaccinated and boosted are at risk of spreading the virus onboard.

“The virus that causes COVID-19 spreads easily between people in close quarters on board ships, and the chance of getting COVID-19 on cruise ships is very high, even if you are fully vaccinated and have received a COVID-19 vaccine booster dose,” the CDC said in its statement.

The CDC also recommended that those who have traveled on a cruise ship who are not vaccinated get vaccinated and receive their booster shot if eligible.

Cruise lines reported 5,013 COVID-19 infections on ships operating in U.S. waters between Dec. 15 and 29, according to CDC data, a significant increase from the 162 cases reported over the prior two weeks.

Many cruise lines have implemented vaccine requirements, and ships have put in place mask rules for common areas as omicron has spread.