Adults face higher risk of severe disease from infections than children’s

The severity of most infectious diseases was found to be at its lowest in school-age children (5-14 years old). Strikingly, the severity was higher among young adults in their 20s than among school-age children for many diseases, including polio, measles, HIV, tuberculosis, typhoid and meningococcal meningitis.

Some infections show a slower rise of severity with age after childhood including COVID-19, SARS, plague and hepatitis A and B but for most infections this rise begins well before old age. Dengue was the only infection that was most severe in school-age children.

These findings suggest that 'immune aging' may start much earlier than previously thought, with the rise in severity of many infectious diseases after childhood apparent by the age of 20 years. The researchers say these results could have major implications for understanding of resilience to infection, optimal vaccine scheduling, drug design and health protection policies over the life course.

We know infants are particularly vulnerable to infectious diseases due to their immature immune system, and the elderly are vulnerable due to immune deterioration. Surprisingly little is known about how the response to infection changes between these age extremes. The finding that 'immune aging' could begin as early as young adults could be a catalyst for much needed new approaches to how drugs and vaccines are designed and scheduled, although this resilience to infection could be attributable to other aspects outside of immune function.

To reveal these patterns of disease severity in different age groups, the researchers collated more than 140 datasets with information on disease severity for over 30 different bacterial and viral infections. These included studies from the pre-antibiotic and pre-vaccine era, to understand natural responses to infection.

They found that, while most diseases have the lowest severity in school-age children, for many the severity increased in young adults. Severity was higher by the age of 20 years for polio, typhoid, tuberculosis, measles, smallpox, chickenpox, HIV, infectious mononucleosis and yellow fever.

Infectious diseases that showed increased severity from age 20 years onwards included Ebola, meningococcal meningitis, cholera, scarlet fever and Lassa fever. Some had a slower rise in severity after childhood, including seasonal flu, brucellosis and hepatitis B acute infection where severe disease was more common from 30 years old.

For SARS, COVID-19, MERS-CoV and hepatitis A disease severity increased from age 40 years. COVID-19 and SARS appear to have more extreme variation in severity by age than other infections, with predominantly very mild disease in children, and high case fatality rates in the elderly.

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Immunogenetics: open access