A nine-year-old boy who contracted Covid-19 in Eastern France did not pass the virus on despite coming into contact with more than 170 people, according to research that suggests children may not be major spreaders of the virus.

The boy was among a cluster of cases linked to Steve Walsh, the Hove-based businessman who became the first Briton to test positive for coronavirus after attending a sales conference in Singapore in January.

Walsh unwittingly passed the infection on when he joined 10 British adults and a family of five at a chalet in the ski resort of Contamines-Montjoie in the Haute-Savoie region after flying in from London.

Most of the chalet guests contracted the virus, but an investigation by Public Health France found that the nine-year-old did not pass it on to either of his siblings nor anyone else, despite coming into contact with 172 people, all of whom were quarantined as a precaution, and having lessons at three separate ski schools.

A report on the investigation published in Clinical Infectious Diseases describes how tests revealed the boy to be infected with Sars-Cov-2, the virus that causes Covid-19, and also influenza and a common cold virus. While both of his siblings caught the latter infections, neither picked up the coronavirus.

“One child, co-infected with other respiratory viruses, attended three schools while symptomatic, but did not transmit the virus, suggesting potential different transmission dynamics in children,” Kostas Danis, an epidemiologist at Public Health France told the French news agency AFP.

The boy had only mild symptoms and when tested was found to have levels of virus that were barely detectable. The low level of infection is thought to explain why he did not infect other people.

The researchers believe that since children typically have only mild symptoms, they may transmit the virus far less than infected adults. “Children might not be an important source of transmissions of this novel virus,” they write.

Why children generally escape the worst of the virus is not well understood, but many scientists suspect that their immune response is somehow able to clear the infections more rapidly than older adults, who tend to be hit much harder by the illness.

The report comes after researchers at UCL concluded this month that school closures would likely have only a small effect on the spread of the virus, and that this should be weighed up against the profound social and economic costs. Dozens of countries have closed their schools to slow the transmission of coronavirus, though the restrictions have been brought in to avoid the social gatherings that happen around schools as well as limiting spread of the virus within them.

The role of children in spreading the virus remains one of the key mysteries of the coronavirus pandemic and the question of whether those who develop few if any symptoms are carriers is still being debated. While the proportion of children who experience severe illness is tiny compared with that of older people, some have fallen seriously ill and died from the infection.

“Better understanding of who is responsible for transmission and when during the disease progression is a really important piece of the jigsaw and we still don’t have any real insight,” said Professor Jonathan Ball, a virologist at Nottingham University. “I keep hearing about significant asymptomatic infection, for example the US Navy personnel, but still have no real idea as to how important they might be with respect to spread.”