I think it depends on how long ago you received your basic vaccine series. All this info is on the SS site:
Up to Date Vaccinated Guests: You will need to be fully up to date with your vaccine status (“up to date” means a person has received all recommended COVID-19 vaccines, including any booster dose(s) when eligible), in accordance with each of the countries you plan to visit on your respective itinerary. We recommend that you refer to each country’s official government website to understand the complete vaccination status requirements for entry, as the ability to roam freely, enter restaurants, cultural institutions, etc. For example, some countries may allow entry for fully vaccinated travellers, but will require “up to date” vaccine status to dine at a restaurant. It is your responsibility to ensure you are adequately inoculated to enter and enjoy the countries on your itinerary.Similar to testing, we have seen that destinations across the globe are now updating validity dates for vaccination series. Many countries have differing rules (both based on residency and as entry requirements), with most now indicating an “up to date” COVID-19 vaccine includes an additional dose anywhere from 90 to 270 days past the original series completion date. Quite simply, the vaccine will no longer be considered up to date unless an additional dose has been received after a certain time has elapsed from the completion of the primary series. So while the booster (or an additional dose) is not necessarily a requirement to sail with us, an up to date vaccination series is. By way of example for our guests residing in the US, please click here to understand the CDC’s definition of “up to date.”