People younger than 20 are around half as susceptible to coronavirus as those aged 20 and older, research suggests.
The modelling study also estimates that clinical symptoms appear in 21% of infections among 10 to 19-year-olds, rising to 69% in adults over 70.
Researchers say understanding the role of age in the transmission and disease severity of Covid-19 is crucial for determining the impact of social distancing interventions as well as accurately estimating the number of cases worldwide.
Rosalind Eggo and Nicholas Davies, from the London School of Hygiene and Tropical Medicine, and colleagues developed an age-based transmission model with data from 32 locations in six countries – China, Italy, Japan, Singapore, Canada and South Korea.
They found that across all regions, people under 20 years of age are about half as susceptible to infection as people aged 20 and older.
For young people aged between 10 and 19, only 21% of those infected experience clinical symptoms.
For adults 70 and over, 69% experience clinical symptoms, the study published in Nature Medicine found.
The researchers also compared an outbreak of Covid-19 with an outbreak of pandemic influenza, to project the impact of school closure.
They say that because children would have a smaller role in transmission than adults, school closures may be less effective against Covid-19 than flu.
However, they add that this does not mean school closures are ineffective.
The authors write: “These results have implications for the likely effectiveness of school closures in mitigating Sars-CoV-2 transmission, in that these might be less effective than for other respiratory infections.
“There are also implications for the global expected burden of clinical cases – countries with a large number of children might need to account for decreased susceptibility and severity in burden projections.”
Researchers also simulated Covid-19 epidemics in 146 capital cities around the world and found that the total expected number of clinical cases in an unmitigated epidemic varied among cities depending on the median age of the population.
They found that there were more clinical cases per capita projected in cities with older populations, and more asymptomatic infections – or infections with mild symptoms – in cities with younger populations.
However, the estimated basic reproduction number – the average number of cases an infected person is likely to cause while infectious – did not substantially differ by median age.
They researchers write: “In countries with younger population structures – such as many low-income countries – the expected per capita incidence of clinical cases would be lower than in countries with older population structures, although it is likely that comorbidities in low-income countries will also influence disease severity.
“Without effective control measures, regions with relatively older populations could see disproportionally more cases of Covid-19, particularly in the later stages of an unmitigated epidemic.”