Patient age is a key determinant in the timing of visits for respiratory illness; pediatric patients, and specifically preschool aged children, three to four years old, seek ambulatory and emergency care earliest. Further, respiratory illness among children less than five years of age is significantly associated with mortality from P&I with a four to five week lead time. Pediatric populations are sentinels of infection and they signal the consequent burden of illness. Though this does not necessarily prove that preschool age children are driving the yearly influenza epidemics, these findings intriguingly suggest that preschool age children are the initial group infected and may be important in the subsequent spread.
There is ample prior evidence that children play a primary role in influenza transmission. Given their increased tendency to acquire and shed influenza, children have been identified as predominant vectors in the household spread of influenza (30
). Our findings support the notion that specifically targeting the preschool children may reduce transmission. Children under five years of age have higher infection rates than older children (33
). In addition, vaccination of this age group has been shown to significantly reduce morbidity among their household contacts (36
). For this reason, concentrating immunization efforts on preschool children may eliminate the primary pathway of infection.
Other studies have shown that older children (5–18 years old) are the most important targets and that their routine vaccination would reduce disease burden across the community level (11
). Our results suggest that younger children may initiate spread to these older children and therefore may be of value as targets of vaccination out of proportion to their lesser numbers.
While our study suggests that young children are infected first, there are other possible explanations for their early presentation to the health care system. It may be not just the inherent vulnerability of children, but also health care seeking behaviors that make them timely sentinels of influenza (41
). Family members may have a lower threshold for bringing in febrile young children because of morbidity concerns specific to the pediatric population and will thus been seen by physicians at the earlier stages of viral illnesses (42
). However, we find that the pediatric ED populations arrive prior to the pediatric ambulatory populations. Because the ED populations are naturally more acutely ill (24
), the reason for the early presentation of children is likely at least partly rooted in genuine morbidity, and not just parental behavior. In addition, if the early arrival children were could be explained primarily by the behavior of worried parents and pediatricians, we would instead expect to see the youngest, most fragile children, infants, arriving before the preschoolers; in preschoolers, simple febrile illnesses simply do not pose the same risks or require as much testing (44
A limitation of our study is that we are measuring respiratory illness, but not virologically confirmed influenza infection. Our findings are confounded by co-circulation with other viruses, for which there are no vaccinations currently available, including respiratory syncytial virus and parainfluenza virus. Another limitation is that our data are from the Greater Boston Area and may not be entirely generalizable to other regions. However, the patients are seen at seven diverse institutions and are likely to be highly representative of the region; also, a priori, it is not clear why there would be regional differences.
This study has other implications as well. Since the data are available in a real time population health monitoring system, understanding the temporal dynamics of respiratory illness through different age groups can be used to inform medical practice and enable improved prevention and control efforts by individual clinicians. Monitoring respiratory illness in the ambulatory care and pediatric ED populations using syndromic surveillance systems was shown to provide even earlier detection and better prediction of influenza activity then the current CDC sentinel surveillance system. Supplying physicians with a mechanism to identify the earliest and most sensitive warning of respiratory mortality can help them implement prevention strategies that will protect their general patient population.
We demonstrate clearly, across a region, that preschool age children are the first to seek healthcare for respiratory infections and further that there is a strong association between their temporal patterns of illness and subsequent mortality in the general population from influenza. While our findings do not definitively indict preschool age children as those initially infected and primarily responsible for spread to other age groups, this age group does appear to have an important role in influenza transmission. These results bolster arguments for a recommendation currently under consideration by the ACIP to begin to universally vaccinate preschool aged children.