Aug 4 (Reuters) - Royal Caribbean Group (RCL.N) on Wednesday flagged a modest hit to its bookings from the Delta variant of the coronavirus and leaned on vaccines for safe voyages as the cruise industry takes to the sea after a pandemic-led standstill. Shares fell about 2.4% after the company, which operates the biggest cruise ships in the world, reported a bigger quarterly loss than expected. Cruise operators resumed sailing from U.S. ports in June with mostly vaccinated passengers and crew on limited capacity following lengthy talks with the U.S. Centers for Disease Control and Prevention about health and safety protocols. The resumption came more than a year after operations were suspended, but a few on-board cases in recent weeks and rising infections in U.S. states have raised worries that a rebound for the cruise industry could be delayed. "The vaccines are the ultimate weapon," Royal Caribbean Chief Executive Richard Fain said on a call with analysts. "It"s impossible to eliminate cases on board a ship totally. But the steps that we are taking are designed to prevent the isolated cases from becoming an outbreak and it seems to be working." Royal Caribbean requires passengers above 12 years of age to be fully vaccinated, except for cruises departing from Florida, due to the state"s ban on businesses asking for so-called "vaccine passports". Florida is crucial for cruise companies as it has the main ports of departure for Caribbean voyages. Still, Royal Caribbean said bookings for 2022 cruises were "practically back" to pre-pandemic levels due to the pent-up demand for vacations. It expects 80% of its capacity to be in service by 2021 end and turn cash-flow positive in about six months. The company reported a second-quarter adjusted net loss of $1.3 billion, or $5.06 per share, bigger than $4.39 loss estimated by analysts, according to Refinitiv IBES data.