One fully vaccinated passenger tested positive for COVID-19 aboard the American Constellation cruise ship that was docked in Haines on Monday. The passenger and his two travel companions, both of whom tested negative, isolated before the ship arrived in Haines. No other passengers tested positive, according to an American Cruise Lines spokesperson, who was unsure whether the traveler was symptomatic. All guests and crew members aboard the ship were vaccinated.

Haines officials announced on Monday that an unspecified number of passengers aboard the ship tested positive for COVID-19 and were in quarantine, while other vaccinated passengers entered town. There was just a single case on the ship.

Vaccination remains effective at dramatically reducing the likelihood of hospitalization and death from COVID-19, according to health officials. As of mid-July, about 97% of people hospitalized with COVID-19 were unvaccinated.

Public health officials are concerned about an ongoing surge in cases nationwide and the rapid spread of the new delta variant. Delta was first identified in Alaska in late May, but by the week of July 11 it accounted for 96% of the state’s new cases that were genetically sequenced, according to the latest analysis by the Alaska Department of Health and Social Services. The new strain “may significantly increase transmissibility and severity of infection in those not vaccinated,” the analysis states.

While researchers are still working to understand details about the delta variant, preliminary data released by the CDC suggest that vaccinated people can, in rare instances, become infected and transmit the strain. The CDC changed its guidelines last week to advise everyone to wear masks indoors in high-risk areas. Haines remains at a “low” level of community transmission. Skagway and Juneau are classified as “high,” according to the CDC.

The American Constellation canceled a Southeast cruise in July after two passengers and a crew member tested positive while the ship was in Petersburg. A total of 16 cases were linked to that ship. At the time, American Cruise Lines did not require its crew to be vaccinated. Since then, the company has changed its protocol and now requires guests and crew members on its Alaska cruises to be fully vaccinated.

Haines was the American Constellation’s last stop before its trip ended on Wednesday in Juneau.