They welcome guests to cruise on as many back-to-back cruises as they wish — including kids under vaccination age. The travel requirements and health protocols are the same as a single cruise, and back-to-back cruisers will need to meet all those requirements. For subsequent cruises, the "pre-cruise test" requirement is fulfilled by a test between cruises. This test will be administered by Royal Caribbean at no expense to the guest, as long as your consecutive cruises are less than 3 days apart.