The US Centers for Disease Control and Prevention (CDC) issued an advisory on Thursday warning people against going on cruises, regardless of their vaccination status.

“It is especially important that travellers who are at an increased risk of severe illness from COVID-19 avoid travel on cruise ships, including river cruises, worldwide, regardless of vaccination status,” said the CDC in its advisory.

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The CDC’s warning comes at a time when there has been a recent spike in COVID-19 cases onboard ships, as the highly transmissible omicron variant wreaks havoc around the world. In cognizance of the same, the CDC has increased its travel alert for ships to the highest level, and is actively monitoring dozens of ships that have reported COVID-19 outbreaks of late.

Between December 15 and 29, cruise ships operating on US waters reported around 5,000 COVID-19 cases, a massive spike from the 162 cases reported in the first two weeks of December. The spike is believed to have been caused by the spread of the omicron variant, which currently accounts for more than 58 percent of COVID-19 cases in the US, as well as people taking cruises for the holiday season. COVID-19 spreads quite easily within confined spaces, such as that of a cruise ship’s interior, and the CDC has said that the risk of contracting the virus on cruises is especially high, even among those who are vaccinated.

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However, if people still decide to go ahead for a cruise, the CDC has advised people to get tested 1 to 3 days prior to the trip and 3 to 5 days after the trip, regardless of vaccination status. It has further said that people planning to take a cruise should get vaccinated (if they have not already taken the vaccine) or take a booster dose if eligible. Travellers have also been strongly advised to wear facemasks at all times in shared spaces, and partially or unvaccinated travellers will have to self-quarantine for five days post-travel.