Cruise lines can soon begin trial voyages in U.S. waters with volunteer passengers helping test whether the ships can sail safely during a pandemic.

The Centers for Disease Control and Prevention gave ship operators final technical guidelines Wednesday for the trial runs. The CDC action is a step toward resuming cruises in U.S. waters, possibly by July, for the first time since March 2020.

A spokeswoman for the cruise industry's trade group said the group was reviewing the CDC instructions.

Each practice cruise — they'll run two to seven days — must have enough passengers to meet at least 10% of the ship's capacity. Volunteers must be 18 or older and either fully vaccinated or free of medical conditions that would put them at high risk for severe COVID-19.

The ship operator must tell passengers that they are simulating untested safety measures "and that sailing during a pandemic is an inherently risky activity," the CDC guidelines state.

Passengers must be examined for COVID-19 symptoms before and after the trip, and at least 75% must be tested at the end.

Restrictions on board will include face masks and social distancing. The CDC will allow guided shore excursions — no wandering about on their own — if tour operators follow certain standards.

Ships must make at least one practice run before resuming regular cruises in U.S. waters, although operators will be able to avoid the requirement if they vouch that 98% of crew and 95% of passengers are vaccinated.