The new guidance applies to older adults, people with certain medical conditions and pregnant and recently pregnant people. Prior to Friday’s announcement, the agency recommended that only people who were not fully vaccinated against COVID-19 avoid cruise ships.

The change comes as the U.S. faces its fourth wave of COVID-19, driven by the delta variant. As of Friday, the country has reported 987,417 new cases and 6,037 virus-related deaths in the past week, according to data from Johns Hopkins University.

The CDC’s website says the virus that causes COVID-19 can spread easily between people in close quarters on ships, and the chance of getting COVID-19 on cruise ships is “high.”