Lots of vulnerable people can't be vaccinated and if they are won't be completely protected so we need to ensure no one else has it for them to catch it
Short of holding people down for a jab that simply won't happen - and that's not even allowing for others entering the country, mutations, doubts about transmission from the vaccinated and all the rest
Nice as it would be it'll never be possible for the vulnerable to be "completely protected", and I'd suggest it's unkind to encourage them to think this way