Aug 20 (Reuters) - The U.S. Centers for Disease Control and Prevention (CDC) said on Friday people at an increased risk for severe illness from COVID-19 should avoid travel on cruise ships, including river cruises, irrespective of their vaccination status. Older adults and people with medical conditions are more likely to get severely ill from the disease and should take professional advise before cruise ship travel, the agency said. Cruise operators have been sailing from U.S. ports again in recent weeks with mostly vaccinated guests and crew after lengthy talks with the CDC. But a few on-board coronavirus cases and a Delta variant-driven increase in U.S. infections have raised worries about the cruise industry"s recovery from the lows seen last year. The health agency in May began approving some cruise operations and in June eased its warnings and recommended only fully vaccinated people take trips. read more Reporting by Manojna Maddipatla and Mehr Bedi in Bengaluru; Editing by Aditya Soni Our Standards: The Thomson Reuters Trust Principles.