Six passengers—four of whom were vaccinated—aboard Royal Caribbean's Adventure of the Seas cruise ship have tested positive for COVID-19, sending shares of all major cruise lines down on Friday.

The seven-night cruise departed from Nassau, Bahamas, on July 25. One of the vaccinated patients is symptomatic, while the other three are asymptomatic. None of the four were traveling together. The two unvaccinated positive cases are minors who were traveling together. The guests were immediately quarantined and contact tracing has taken place, the cruise line says.

Still, the news sent shares of cruise ships down. Royal Caribbean, Carnival and Norwegian Cruise Lines were all down roughly 4% as of 12:30 p.m. ET.

The incident is once again fanning fears among investors for the cruise industry, which sat in dry dock for 15-months due to the pandemic. It was on board a Princess Cruises ship—the Diamond Princess—that 712 people became infected in one of the first major outbreaks outside of China. Passengers were quarantined on the ship beginning Feb. 1 and many who were not infected were not permitted to leave their cabins until Feb. 17 or Feb. 19.

The cruise industry has since made several changes to how it handles passengers, ranging from reduced capacity to changes to the buffets. Still, bookings are currently slow, even as Americans begin venturing out and actively vacationing in 2021.

This story was originally featured on Fortune.com