According to their website:
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Vaccination: For departures on or before October 31, 2023, Viking will continue to require that all guests be vaccinated against COVID-19. We suggest carrying your physical vaccine records at all times.Viking will not require COVID-19 vaccination for voyages departing on or after November 1, 2023. For added protection, we recommend that guests stay up to date with all relevant vaccines, including COVID-19, in accordance with their healthcare provider. Plese Note: Select countries continue to have COVID-19 related travel requirements. You will receive details via e-mail ahead of your departure.
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https://www.vikingcruises.com/oceans/why-viking/health-and-safety/health-and-safety-faq.html