U.S. cruise liner Royal Caribbean Group said on Friday only passengers with a negative COVID-19 test will be let aboard, after six guests on its Adventure of the Seas cruise ship  tested positive during routine testing. The rule will apply to all guests departing on cruises that are five nights or longer, irrespective of their vaccine status, the company said. Of the passengers who tested positive, only one had mild symptoms. Four out of the rest were vaccinated and not traveling together, while two were unvaccinated minors. The guests were immediately quarantined and their travel parties and all close contacts were traced and tested negative, the company said. The guests and their travel parties will disembark later today and travel home. The Adventure of the Seas cruise, which set sail from Nassau in the Bahamas on July 24, required all travelers aged 16 or older to be fully vaccinated and test …