Anyone going on a Saga holiday or cruise in 2021 must be fully vaccinated against Covid-19, the tour operator has said. Saga, which specialises in holidays for the over-50s, said it wanted to protect customers' health and safety. The firm said it would delay restarting its travel packages until May to give customers enough time to get jabs.
Doesn't say how you'd prove it
https://www.bbc.co.uk/news/business-55738918