People should take precautions when hugging others, says an expert, amid expectations of a further easing of England's Covid restrictions this week.
Prof Cath Noakes said hugs should be selective, short, and avoid face-to-face contact.
Cabinet Office minister Michael Gove said No 10 wants to see "friendly contact" between people restored.
He said the PM will on Monday confirm an easing of England's rules from 17 May.
Under the next stage of the government's roadmap for lifting England's lockdown, people can meet in groups of up to 30 outdoors, while six people, or two households, can meet indoors.
People will also be allowed to stay overnight with those not in their household or bubble. Pubs, restaurants and other hospitality venues will be permitted to reopen indoors.
Some foreign holidays are set to be allowed. On Friday, the government announced that 12 countries would be added to England's travel green list, meaning anyone returning will not need to quarantine from 17 May.
On Sunday, the UK recorded another two deaths within 28 days of a positive test and a further 1,770 cases.
Prof Noakes, a member of the Sage committee that advises the government, said the risks of grandparents who are fully vaccinated hugging their grandchildren are likely to be low in most cases.
Speaking in a personal capacity, she said it would worry her "if we were advocating we could hug all of our friends every time we meet them again" as it would "perpetuate an awful lot of additional close contact that could spread the virus".
"The reality is that when you hug someone you are very close to them and we know the virus is in people's breath and you are very close to that breath at that moment."
She advised that if people are going to hug others, it should be restricted "to very small numbers of close family who perhaps you really value a hug from.
"I think don't hug too frequently, keep it short, try and avoid being face to face, so perhaps turn your face away slightly, and even wearing a mask could help," she said.
Prof Noakes warned that, even after vaccination, someone could get infected and could transmit it to others while unaffected themselves,
"So that's why we still need to be a bit cautious for a while yet. We've come a long way with this. The virus, although it's now very low prevalence, hasn't gone away."
The scientists and engineers advising the government realise that their caution about hugging might not be popular.
They know how many people see hugs as an essential form of bonding with their nearest and dearest.
But they've also studied how the virus spreads - how an infected person releases it in their breath.
That means that if someone gets very close, there's an increased chance of them inhaling the infectious particles floating in the air.
The tips for reducing those risks won't be popular - they go right against the grain of human instinct.
Basically, hug as few people as possible and don't linger over it. And maybe wear a mask if someone is vulnerable.
That's why the prime minister talks of us using common sense about this.
But in the excitement of reunions, after so long apart, that might not be easy.
Current social distancing rules advise people to stay 2m (6ft) away from those they don't live with, or are in a social bubble with.
In England, if it's not possible to stay 2m apart, the "one-metre plus" guidance means people should be at least 1m (3ft) apart with extra precautions in place (such as face coverings).
Earlier, Mr Gove told the BBC's Andrew Marr show that "all being well" Prime Minister Boris Johnson would confirm on Monday that there will be a relaxation of restrictions starting 17 May.
He also hinted that closer contact between family and friends could soon be allowed.
"Without prejudice to a broader review of social distancing, it is also the case that friendly contact, intimate contact, between friends and family is something that we want to see restored," he said.
'Time is right'
Meanwhile Prof Andrew Pollard, director of the Oxford vaccine institute, told Andrew Marr that he believed it is the right time to ease England's restrictions.
Asked whether it would be safe for family and friends to hug on 17 May, he said the country has seen the "extraordinary success" of both the Covid vaccine rollout and the "prolonged" lockdown.
"I think it is time to start, based on the very careful modelling that's been done, relaxing some of those restrictions," he said.
"That means we're in a very fortunate position here in the UK."
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