How does age relate to COVID-19 rates and symptom severity?

People under 20 years of age are approximately half as susceptible to COVID-19 compared to people 20 years of age and older, according to a modelling paper published in Nature Medicine.

The study, carried out by modellers from the London School of Hygiene & Tropical Medicine (LSHTM), also estimates that clinical symptoms appear in 21% of infections among 10- to 19-year-old people, rising to 69% in adults over 70 years of age.

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.

A markedly low proportion of COVID-19 cases have been reported among children, which could be explained by children’s having lower susceptibility to infection, a lower propensity to show clinical symptoms or a combination of both, compared with adults.

Dr Rosalind Eggo and colleagues developed an age-based transmission model with demographic data from 32 locations in six countries ― China, Italy, Japan, Singapore, Canada and South Korea ― and data from five published studies on estimated infection rates and symptom severity across different age groups. Using the model, the authors were able to simultaneously estimate disease susceptibility and clinical symptoms by age.

Dr Eggo, Assistant Professor at LSHTM, said: “We analysed COVID data to try to understand why we see the low number of reported cases in younger ages. The study found that not only are younger ages less likely to show clinical symptoms of infection, but also that they were less likely to become infected in the first place. Quantifying the role of different age groups in transmission is important for designing intervention strategies against COVID epidemics, as well as making projections for what might happen next.”

The authors 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.

Countries with younger populations — such as many low-income countries — might have fewer incidences of infection per capita, but the authors note that comorbidities associated with low-income countries might also influence disease severity.

The researchers argue that further research is needed to determine the transmissibility of asymptomatic infections or infections with mild symptoms in order to effectively forecast and control COVID-19 epidemics.

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