Introduction
School-aged children are often recognized as primary drivers of influenza transmission within communities1, and in the fall of 2009 school reopening dates were associated with the local surges of pandemic influenza2. Children frequently have larger social networks3,4, experience prolonged viral shedding5, have lower coverage rates for influenza vaccine6, and may lack sufficient preexisting immunity for herd effects7. Although most of the frequent influenza infections among school-aged children are mild to moderate, some children can still develop serious influenza-related complications following infection8. During the 2017-18 influenza season, there were an estimated 11.5 million cases of influenza in children and over 48,000 pediatric hospitalizations in the U.S. alone8.
The rapid evolution and wide variability of the influenza virus contributes to the challenges of control. Normal efforts in disease prevention, such as vaccination, are hampered as vaccines must be updated and administered annually to account for changes in circulating viruses9, leading to varying levels of effectiveness from year to year10. Thus, it is important to consider alternative strategies to control outbreaks, especially during seasons when vaccine effectiveness is suboptimal, or when a well-matched vaccine is not yet available (e.g., in early stages of a pandemic).
School closures include planned breaks in instruction for holidays or teacher training, and unscheduled breaks due to weather, safety or other emergencies. With regard to their anticipated effects on influenza transmission, school closures are considered a nonpharmaceutical intervention (NPI) only when implemented sufficiently early relative to the start of an outbreak (i.e., before influenza becomes widespread in schools and surrounding communities)11. Effectiveness of preemptive school closures has been extensively studied and scrutinized in systematic literature reviews12,13. In contrast, reactive school closures—implemented only after influenza is widespread in schools—are not considered NPI, but rather a consequence of the disease11 because epidemiologic studies have not found them to effectively reduce medically attended influenza (MAI) in surrounding communities14-16. Studies noted reactive closures to have no statistically significant impact on overall influenza-like illnesses (ILI) rates14,15. In fact, these unplanned closures often have socioeconomic consequences and may further introduce challenges to households, such as making alternative childcare arrangements and loss of access to school lunch programs16.
Schools close for regularly scheduled or planned breaks (holidays) throughout the academic year (Figure 1). At least one earlier study reported that such planned school breaks may interrupt the dynamics of seasonal influenza by changing social contact patterns among children17. Such closures have been associated with a reduction in the reproduction number relative to when school is in session, leading to reduced transmission18,19. However, the precise impact of these breaks on seasonal influenza remains unclear. Few studies have investigated the effect of planned school closures on local transmission, and no studies currently assess the impact of numerous breaks within an academic year to account for potential seasonal differences in the timing of circulation.
To account for multiple breaks and seasonal timing, we investigated the role of regularly scheduled school breaks on ILI within a single school district over the course of five academic years. We assessed rates of ILI-related absenteeism (a-ILI) during two-week periods leading up to and following scheduled winter and spring breaks.