A young human being below the age of puberty or below the legal age of majority. For many young children with flu, it is the first time they are exposed to the virus. Their immune system is naïve to influenza and therefore responds more slowly to the infection. This means the influenza virus can cause significant ill-effects before the immune system can bring it under control.

Source: Kids are more vulnerable to the flu – here’s what to look out for this winter, National Centre for Immunisation Research and Surveillance (NCIRS)