Fever Responses Are Enhanced with Advancing Age during Respiratory Syncytial Virus Infection among Children under 24 Months Old.

The most important risk factor for severe respiratory syncytial virus (RSV) infection is considered young age due to the immature immune system. The risk at young age is reported greater for RSV than for other respiratory infectious agents. Based on the strong association between young age and severity of RSV infection due to immature immunity, we aimed to assess whether there were any age-related differences in fever responses, as one clinical aspect of the immune response. In our observational study over two seasons (2014-2015 and 2015-2016), daily body temperatures of children under 3 years old with RSV infection were recorded from the first medical visit during the acute phase to defervescence. The body temperature records were analyzed among 171 children of four age groups (< 6, < 12, < 24 and ≥ 24 months), in terms of fever development, degrees of fever onset, the highest fever during the period, and fever duration. There were 54 patients in the group of < 6 months, 41 in the group of < 12 months, 58 in the group of < 24 months, and 18 in the group of ≥ 24 months. We thus found the correlation between age and fever responses under 24 months old; namely, the more the age advanced, the more frequently high and prolonged fever was experienced. Importantly, infants under 6 months old tend to show the suppressed fever responses. In conclusion, young infants with reduced fever response during RSV infection do not implicate less severity and needs attentive management.

Age-Specific Profiles of Antibody Responses against Respiratory Syncytial Virus Infection.

Respiratory syncytial virus (RSV) is one of the most prevalent causative agents of lower respiratory tract infections worldwide, especially in infants around 3 to 4months old. Infants at such a young age have maternally-transferred passive antibodies against RSV but do not have active immune systems efficient enough for the control of RSV infection. In order to elucidate age-specific profiles of immune responses against RSV protection, antibody responses were examined by using blood samples in both acute and convalescent phases obtained from child patients and adult patients. In addition to the serum neutralization activity, antibody responses to the RSV fusion protein (F protein) were dissected by analyzing levels of total IgG, IgG subclasses, the binding stability, and the levels of antibody for the neutralization epitopes. It was suggested that children's antibody responses against RSV are matured over months and years in at least 5 stages based on 1) levels of the neutralization titer and IgG3 for F protein in the convalescent phase, 2) geometric mean ratios of the neutralization titers and levels of IgG1 and IgG2 for F protein in the convalescent phase compared to those levels in the acute phase, 3) the affinity maturation of IgG for F protein and the cross reactivity of IgG for RSV glycoproteins of groups A and B, 4) levels of neutralization epitope-specific IgG, and 5) augmentation of overall antibody responses due to repetitive RSV infection.