Clinics (Sao Paulo). 2020;75:e1947.

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Why is SARS-CoV-2 infection milder among children?

Patricia Palmeira , José Alexandre M. Barbuto , Clovis Artur A. Silva , Magda Carneiro-Sampaio

DOI: 10.6061/clinics/2020/e1947

Acute respiratory viral diseases, like most other infectious diseases, affect mainly infants and young children, who present with 6-8 episodes per year and usually develop more severe manifestations than in adults. It is well-documented that innate antiviral immune responses in early life are characterized by lower type I interferon (IFN) responses () and reduced natural killer cell activity, which despite their higher numbers exhibit lower cytotoxic capacity and reduced number of cytoplasmic granules and degranulation ability (). Additionally, lower cytotoxic T lymphocyte activity and numbers of effector and memory CD8⁺ T cells have also been described at younger ages ().

Most data indicate a significant role for innate immunity and T-cell cytotoxicity in the control of viral infections. Surprisingly, however, as seen in the severe acute respiratory syndrome-related coronavirus (SARS-CoV) () and Middle East respiratory syndrome-related coronavirus (MERS) () outbreaks, the current SARS-CoV-2 pandemic shows low morbidity and near-absent mortality in previously healthy children. On February 28, 2020, in one of the first publications on the clinical features of SARS-CoV-2 infection, Guan et al. () analyzed 1,099 laboratory-confirmed patients from Wuhan, China. Among these, only nine were under 14 years (0.9%) and only one had a severe course. Shortly thereafter, a review of 72,314 cases, conducted by the Chinese National Center for Disease Control and Prevention, showed that less than 1% of cases were in children under 10 years of age (). Similarly, reports from Italy, Brazil, and the USA confirm a lower incidence of serious infections among younger individuals (-).

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