Why Does the Severity of COVID-19 Differ With Age?: Understanding the Mechanisms Underlying the Age Gradient in Outcome Following SARS-CoV-2 Infection

Pediatr Infect Dis J. 2022 Feb 1;41(2):e36-e45. doi: 10.1097/INF.0000000000003413.


Although there are many hypotheses for the age-related difference in the severity of COVID-19, differences in innate, adaptive and heterologous immunity, together with differences in endothelial and clotting function, are the most likely mechanisms underlying the marked age gradient. Children have a faster and stronger innate immune response to SARS-CoV-2, especially in the nasal mucosa, which rapidly controls the virus. In contrast, adults can have an overactive, dysregulated and less effective innate response that leads to uncontrolled pro-inflammatory cytokine production and tissue injury. More recent exposure to other viruses and routine vaccines in children might be associated with protective cross-reactive antibodies and T cells against SARS-CoV-2. There is less evidence to support other mechanisms that have been proposed to explain the age-related difference in outcome following SARS-CoV-2 infection, including pre-existing immunity from exposure to common circulating coronaviruses, differences in the distribution and expression of the entry receptors ACE2 and TMPRSS2, and difference in viral load.

Publication types

  • Review

MeSH terms

  • Adaptive Immunity*
  • Adult
  • Age Factors*
  • Angiotensin-Converting Enzyme 2 / metabolism
  • Blood Coagulation / immunology
  • COVID-19 / immunology*
  • Child
  • Cross Protection
  • Cross Reactions
  • Endothelium / immunology
  • Humans
  • Immunity, Heterologous*
  • Immunity, Innate*
  • Patient Acuity
  • SARS-CoV-2 / immunology*
  • Serine Endopeptidases / metabolism
  • Viral Load / immunology


  • Angiotensin-Converting Enzyme 2
  • Serine Endopeptidases
  • TMPRSS2 protein, human