Grandparents who have received the Covid vaccine should not yet hug their families, a professor has warned, amid fears that those who have had the jab will abandon the lockdown rules.
Professor Janet Lord, director of the Institute of Inflammation and Ageing at the University of Birmingham, urged ongoing caution even as numbers of those vaccinated increased.
Asked whether people who had received the jab could hug their children, she told BBC Radio 4's Today programme: "I would certainly advise not to do that at the moment because as you probably know with the vaccines they take several weeks before they are maximally effective.
"It's really important that people stay on their guard even if they've had that first vaccination.
"If people do relax what they're doing then it reduces the benefits of the vaccination."
Responding to a survey about public compliance with coronavirus regulations after having received a vaccination, Prof Lord continued: "That's the worrying thing about the idea of a (coronavirus immunity) passport.
"People might think (it is a) passport to freedom and even those who haven't been vaccinated will see those changing their behaviours and think, 'Well why should I bother if no one else is either?'
"That's the real worry we've got at the moment."
It comes as ministers were warned that millions of people are likely to begin ignoring Covid restrictions once they have been vaccinated.
Minutes of meetings held by the Scientific Advisory Group for Emergencies (Sage) cite a survey which says that 29 per cent of people will adhere to restrictions less strictly once they have had a vaccine, while 11 per cent will "probably no longer follow the rules".
Papers released by Sage reveal concerns that changes in the behaviour of those who get the jab could more than "offset" the benefits of the vaccination programme over the next few months.
Professor Janet Lord told the Today programme that she would be "happier" with so-called "vaccine passports" if more was known about the effects of the vaccination on virus transmission, but warned there may be other "practical issues".
"What about the practicalities, do you have some sort of large badge on your jacket that says 'I'm vaccinated?"' she said.
"It's people observing you, so if you're going around, you're no longer wearing a mask, you're hugging anyone you feel like hugging then it's the message that it gives psychologically and motivationally that could be the risk."