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It is now just over a week before we are all allowed to mix in our Christmas coronavirus bubbles.

People moving between tiers, families huddling together indoors, the young hugging the old.

It is the perfect petri dish for a virus that thrives on social contact.

COVID Christmas rules: What's allowed during the festive season?

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And the person most likely to bring COVID-19 into the home is not the student back from university, but their younger sibling who is still at school.

Data from the Office for National Statistics show that about one in 50 secondary school pupils is infected - twice the rate of adults over the age of 25.

And while lockdown slowed the spread of the virus in older groups, it had little impact on rates in children, who continued to go to school.

There seems little doubt that transmission is occurring at school, or through social interaction afterwards.

It is worth saying that rates in primary-age children are much lower. They seem less susceptible to the virus, even though social distancing is much more difficult to enforce with young children.

If they only spread the virus to their peers, there would be little problem - they are highly unlikely to become seriously ill unless they have a severe underlying health condition.

But they do spread the virus within the home, putting older members of the household - perhaps grandparents - at risk.

There is a broad consensus that closing schools in the first national lockdown did huge damage to the development and education of children. Some will never catch up.

So schools have been made a priority this term, even if that has meant other parts of society and the economy take a harder hit to try to balance the overall spread of the virus.

The question is what to do now.

The government is rolling out mass testing to schools in areas of London, Essex and Kent with high infection rates.

The tests reduce risk, but do not eliminate it. The experience from Liverpool is that they pick up some, but not all, asymptomatic infections.

So while they can shut down some routes of transmission if people self-isolate, they can also give false reassurance to those wrongly told they are negative.

The London mayor Sadiq Khan - who has told Sky News that a decision on moving London into Tier 3 could be made today - has gone further, urging schools in the capital to close early for Christmas to reduce the risk of transmission between children.

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That makes good scientific sense, and the government accepted the principle with university students.

They were told to go home in early December, after having a COVID test, so there would be at least a fortnight for them to self-isolate before they mixed with their extended family.

If schools stay open until the end of the week, children could be at their peak infectivity just as the Christmas travel window opens.

Boroughs such as Greenwich and Islington are moving learning online this week, and others will surely follow.

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If children have access to computers, that seems a good compromise between education and health.

The vaccine has not come quite soon enough to protect older generations this Christmas.

But closing schools early might just help.