Infants are more susceptible to COVID-19 than children

Angiotensin-converting enzyme 2 (ACE2) has been found to mediate the host cell entry of SARS-CoV-2 that causes COVID-19. However, the link between ACE2 and the observed susceptibility of SARS-CoV-2 infection remains elusive. In contrast, observational studies can help identify the susceptibility biomarker of SARS-CoV-2 infection, those associated with age for example. Data of all PCR tests performed in the state of Sao Paulo of Brazil were gathered from the government database and were analyzed using multivariate logistic regression. Adjusted odds ratios for positive test results were calculated with the adjustment of age, gender, and comorbidities. Over 1.7 million test results were included in the study of which 38% were positive. Elderly was most vulnerable to SARS-CoV-2 infection. While underages were less susceptible than adults aged below 60 years, susceptibility was not equal among different pediatric groups. It is found that age and susceptibility to SARS-CoV-2 infection are not inversely related, but U-shaped, with infants more susceptible than children. Biomarkers that are linearly associated with age cannot explain the reduced susceptibility in children. These include lymphocyte count and cross-reactive antibodies against other coronaviruses that offers cross-protection. The expression level of ACE2 may still be able to explain but further investigations are needed.