Early evidence for effectiveness of statewide school closure for mitigation of influenza

Background: School closures were mandated in response to COVID-19 as a nonpharmaceutical intervention. Infl uenza A was widespread in Wisconsin at the time of mandate, March 18, 2020, allowing its evaluation for infl uenza interruption. Methods: Daily percent positivity of infl uenza A among primary care patients who received a rapid infl uenza diagnostic test was evaluated throughout two infl uenza seasons while schools were open and closed. Results: Median deviation from baseline of percent positivity was signifi cantly lower when schools were closed than when open (-2.25% vs 1.08%; H=7.84; P=0.005). Median deviation in percent positivity for the eight days following statewide school closure was -20.0. Discussion: Unprecedented closures of all Wisconsin schools was associated with a rapid decline in infl uenza A. Additional evaluations within other jurisdictions are warranted. Research Article Early evidence for effectiveness of statewide school closure for mitigation of infl uenza Jonathan L Temte1*, Cecilia He2 and John Tamerius2 1Department of Family Medicine and Community Health, School of Medicine and Public Health, University of Wisconsin in Madison, WI 53715, USA 2Quidel Corporation in San Diego, CA 92121, USA Received: 26 May, 2021 Accepted: 15 June, 2021 Published: 16 June, 2021 *Corresponding author: Jonathan L Temte, MD, PhD, Department of Family Medicine and Community Health, School of Medicine and Public Health, University of Wisconsin in Madison, WI 53715, USA, E-mail:


Introduction
School closure is an important component of Nonpharmaceutical Interventions (NPI) for pandemic infl uenza [1]. Given similarities in transmission dynamics between SARS-CoV-2 and infl uenza, school closure was implemented rapidly as a countermeasure for COVID-19 across the United States Throughout early March, our surveillance team was monitoring rising rates of infl uenza A(H3N2) following an unusual year in which Wisconsin had experienced successive waves of infl uenza B and infl uenza A(H1N1) [3]. Evaluation of medically attended, laboratory-confi rmed infl uenza (MAI), through a Centers for Disease Control and Prevention program [4], was hampered, however, by rapidly changing approaches to care in surveillance clinics during the week of March 9, 2020.
We were able to continue our infl uenza surveillance efforts using a surveillance system based on automated reporting of infl uenza A and B results from rapid infl uenza diagnostic tests in primary care practices [5].
The widespread and abrupt closure of schools across Wisconsin provides an opportunity to evaluate the potential benefi t of proactive school closure for SARS-CoV-2 on MAI [6]. Whereas school closure is considered an important NPI for mitigating the effects of pandemic infl uenza [1], less evidence exists for its role in controlling outbreaks of seasonal infl uenza.
The objective of this study was to assess the potential role of school closures on an independent marker of seasonal infl uenza across two academic years and ending with widespread school closures across one state.

Methods
We used data from Quidel's Virena surveillance platform study was defi ned as any weekday on which school was closed during the study periods. Weekend days were included only if they were continuous with a weekday closure. This allowed a separation from "usual" school attendance and "altered" due to closure. In addition, a weekend that is continuous with one or more closure day(s) can potentially interrupt infl uenza transmission due to serial interval of infl uenza [10].
We compared the daily percent positivity of infl uenza A to a preceding 21-day baseline for percent positivity, with a one week washout period. Accordingly, we summed the number of positive tests over the three week period, occurring from 28 to 8 days before the day of interest, and divided by the total tests performed during this period to provide the baseline. We then subtracted the baseline estimate from the daily estimate of percent positivity to assess the deviation from the baseline.
Because deviations from baseline were not normally distributed, we compared the median deviations from baseline for school closure days to non-closure days using the Kruskal-Wallis statistic. In addition, we introduced a fi ve-day lag to assess potential delayed effects of reduced transmission in school settings, thus allowing for one to two serial intervals of infl uenza [10]. Finally, we assessed the mean deviation from baseline of the last nine days of data following the statewide school closure for SARS-CoV-2 mitigation.

Results
Over the entire surveillance period, 15 found to have little effect on ILI [12], while a study in Japan found school closures reduced the number of infected students by 24% at its peak [13]. These closures have also shown to reduce contact patterns, suggesting capability to mitigate infl uenza spread [14,15]. There are further controversies and limited evidence for the timing and length of closures [6], thus making inferences about the precise impact of school closure challenging.
School closure represents an extreme form of social distancing within a young population with high likelihoods of person-to-person transmission of respiratory viruses. As most states mandated school closure during the week of March 16, 2020 [2], an excellent opportunity exists to evaluate the roles of school closure on transmission of infl uenza and other respiratory viruses. Such assessment, however, may fail to elucidate potential impacts of school over small geographical range and over varying time frames. We therefore encourage additional evaluations of the potential effects of school closure taking into account both the scale (statewide, regional and local) and timing (scheduled vs. unscheduled; proactive vs. reactive) of closure.

Implications for policy & practice
Nonpharmaceutical interventions, including school closure, are essential elements of pandemic infl uenza response.
Unprecedented school closures due to SARS-CoV-2 present a unique opportunity to examine their effects on seasonal infl uenza detection rates within communities. We took advantage of an existing surveillance system; others may be able analyze similar data to determine the effects of statewide school closure on the level of infl uenza.
Planned and unplanned school closures may reduce the number of infl uenza cases in communities. This information could help public health experts and school administrators develop protocols for strategically-timed school breaks to mitigate infl uenza and other respiratory viruses.

Financial and Other Disclosures
This study was part of Ms. He's MPH capstone project at the University of Wisconsin School of Medicine and Public Health. Dr. Temte has received past research support from Quidel Corporation.