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.