Infants are far more vulnerable than older children to severe illness from viruses like influenza, respiratory syncytial virus (RSV), human metapneumovirus (hMPV), and SARS-CoV-2, the virus that causes COVID-19, according to a study yesterday in Pediatrics. 

The findings also highlight significant differences in how respiratory infections affect infants, ranging from asymptomatic or mildly symptomatic infections managed at home to severe illness requiring hospital or emergency department (ED) care. 

For the study, researchers led by a team from Cincinnati Children's Hospital Medical Center followed up with 1,223 healthy children from birth to age 5 years from November 2019 to August 2025. After collecting weekly nasal swabs from participants and pairing the laboratory results with biweekly symptom surveys, the team identified 15,898 acute respiratory infections (ARIs) caused by different pathogens.

Overall, more than three-quarters (78.4%) of symptomatic respiratory infections were managed at home, and the remaining 21.6% were treated in a healthcare setting. 

The researchers identified asymptomatic infections for all respiratory pathogens, including influenza (7.6%) and RSV (10.2%). Infants younger than 1 year experienced the most asymptomatic flu and RSV infections—14.5% and 12.5%, respectively. 

No symptoms were reported in one-fifth of SARS-CoV-2, adenovirus, and endemic coronavirus infections or in one-third of rhinovirus and bocavirus infections.

While infants experienced the highest rates of asymptomatic infections, they were simultaneously at highest risk for serious illness. Compared with older children, infants were significantly more likely to experience respiratory distress or require ED or hospital care after infection with influenza A viruses (odds ratio [OR], 4.73), RSV A (OR, 2.52), RSV B (OR, 2.22), hMPV (OR, 2.36), and SARS-CoV-2 (OR, 3.99). 

The study also looked at gastrointestinal symptoms. Vomiting or diarrhea accompanied about one-third of influenza infections and more than one-quarter of RSV infections, regardless of age.

The authors conclude that the variation in how children respond to the same respiratory viruses could provide important clues to improving pediatric preventive and therapeutic care.

In an accompanying commentary, Leigh M. Howard, MD, MPH, of Vanderbilt University Medical Center, and Jennifer E. Schuster, MD, of Children's Mercy Hospital in Kansas City, Missouri, note the importance of having a full accounting of viral infections in children, regardless of whether they produce symptoms. 

It may be important to incorporate environmental modifications (eg, air and surface cleaning) in addition to hand hygiene to complement other infection prevention measures.

“Understanding the complete spectrum of disease associated with specific viral infections, including those that are asymptomatic or only mildly symptomatic, could provide important insights into estimating vaccine or nirsevimab [an RSV antibody shot] impact, community and household transmission patterns, and the socioeconomic costs of ARI that complement and enhance previous estimates largely derived from health care settings,” they write.

Howard and Schuster also point out that the findings highlight the need for infection-control measures that go beyond isolation and social distancing. Because so many children with ARIs don’t have symptoms, “it may be important to incorporate environmental modifications (eg, air and surface cleaning) in addition to hand hygiene to complement other infection prevention measures.”