At first it was only elderly people. The narrative about coronavirus, fanned by the details of every sad death announced, was that the virus was mainly a concern for those over 70, or people with serious underlying health conditions. These were the groups initially urged to socially distance themselves. But we’re beginning to see that coronavirus can make some younger people seriously ill. Crucially, although the majority of younger people and children will develop symptoms little worse than flu, they could be invisible carriers of the virus – and play a key role in its spread.

When you can’t easily tell if someone has a cold or coronavirus, infection control is far more difficult

Compared to other European countries, the UK was slow in closing schools – waiting until there had already been 104 deaths due to coronavirus to take action. Studies from China showed children were rarely diagnosed with novel coronavirus, and therefore presumably had little role in the spread of the disease.

But most of the diagnostic efforts in China, especially in the early phases of the outbreak, were heavily skewed towards hospital admissions. The majority of hospitalised cases involved adult patients. Did this mean that children and young people weren’t infected, or merely that they suffered more mild symptoms, which were going undetected?

Like many of my colleagues, I could see no obvious reason why children weren’t being infected: this was a virus spread by the respiratory route, not through a process unique to adults. If significant numbers of children were infected and suffered very minor cold-like symptoms, then their potential to spread the virus was immense.

One of the key differences between the novel coronavirus and Sars and Mers is that people with Covid-19 may have only mild symptoms – making it more difficult to detect. With Sars, most people infected quickly developed pneumonia. Identifying and isolating people with severe symptoms is a relatively easy task, and this made infection control through case identification and contact tracing comparatively easy.

The current coronavirus is different. In more than 85% of confirmed cases symptoms can go undetected or be easily confused with the common cold of mild flu. Its biggest weapon – the thing that has, according to one study, allowed it to spread so easily – is this ability to cause mild disease in the majority of people it infects. When you can’t easily tell if someone has a cold or coronavirus, case identification and infection control are far more difficult.

Far from being uninfected by this virus, children could in fact be its unseen carriers, important links in community transmission chains. But there’s still an important piece of the jigsaw missing. While many of us suspect that children have a key role to play in this outbreak, hard evidence to support this belief is still lacking.

In perhaps the largest study of its type, Chinese researchers examined infection outcomes in more than 2,000 confirmed or suspected childhood cases of Covid-19. Just over half of the children had mild cold-like symptoms, or no symptoms at all.

Severe and critical diseases, where body oxygen levels are low and various organs are under threat, were seen in around 5% of the children studied, with the youngest (under one year old) most at risk. There are still significant gaps in this analysis. Importantly, most of these were suspected rather than confirmed Covid-19 cases. But this study does at least reaffirm that most infections in children are mild.

Judging from past pandemics, school closures can be an effective way to reduce the spread of a virus – particularly when they form part of a larger programme of social distancing measures. Every school day, children congregate en masse, often in close proximity, and then return home, taking with them any new infection they’ve picked up. Intuitively, reducing this cycle should help slow the virus.

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By assuming that the young and healthy weren’t at risk, the UK government may have underestimated the effects of coronavirus. Indeed, reports of hospital admissions show that even among younger people this is a virus that can cause serious disease. A study from the US shows that 38% of 508 patients hospitalised with coronavirus were aged between 20 and 54 (children under 19 accounted for less than 1% of the total admissions). Of these 508 patients, 121 were admitted to intensive care; nearly half of this group was under 65.

As with most things in this pandemic, the idea that coronavirus only threatens older people is an oversimplification. Younger people and children are less likely to die from coronavirus, but their mild symptoms could make them contagious carriers of the virus – and they are more difficult to spot.

• Jonathan Ball is professor of molecular virology at the University of Nottingham