Developing guidelines for school closure interventions to be used during a future influenza pandemic

Abstract

Background: The A/H1N1 2009 influenza pandemic revealed that operational issues of school closure interventions, such as when school closure should be initiated (activation trigger), how long schools should be closed (duration) and what type of school closure should be adopted, varied greatly between and within countries. Computer simulation can be used to examine school closure intervention strategies in order to inform public health authorities as they refine school closure guidelines in light of experience with the A/H1N1 2009 pandemic.

Methods: An individual-based simulation model was used to investigate the effectiveness of school closure interventions for influenza pandemics with R0 of 1.5, 2.0 and 2.5. The effectiveness of individual school closure and simultaneous school closure were analyzed for 2, 4 and 8 weeks closure duration, with a daily diagnosed case based intervention activation trigger scheme. The effectiveness of combining antiviral drug treatment and household prophyaxis with school closure was also investigated.

Results: Illness attack rate was reduced from 33% to 19% (14% reduction in overall attack rate) by 8 weeks school closure activating at 30 daily diagnosed cases in the community for an influenza pandemic with R0 = 1.5; when combined with antivirals a 19% (from 33% to 14%) reduction in attack rate was obtained. For R(0) > or = 2.0, school closure would be less effective. An 8 weeks school closure strategy gives 9% (from 50% to 41%) and 4% (from 59% to 55%) reduction in attack rate for R(0) = 2.0 and 2.5 respectively; however, school closure plus antivirals would give a significant reduction (approximately 15%) in over all attack rate. The results also suggest that an individual school closure strategy would be more effective than simultaneous school closure.

Conclusions: Our results indicate that the particular school closure strategy to be adopted depends both on the disease severity, which will determine the duration of school closure deemed acceptable, and its transmissibility. For epidemics with a low transmissibility (R(0) < 2.0) and/or mild severity, individual school closures should begin once a daily community case count is exceeded. For a severe, highly transmissible epidemic (R(0) > or = 2.0), long duration school closure should begin as soon as possible and be combined with other interventions.

Figure 1

Final illness attack rate of epidemics with school closure operational issues. Outcomes of two different types of school closure intervention strategies (individual school closure and simultaneous school closure) for 2, 4 and 8 weeks school closure duration with/without antiviral treatment plus household prophylaxis (T+H) for three different R₀ values of 1.5, 2.0 and 2.5 as a function of a number of daily diagnosed cases (activation trigger) are shown. The outcomes are reported in cumulative illness attack rate as a percentage of the simulated population size. The non-intervention or baseline epidemics are shown in green line. Three different colours have been used to report school closure periods; dark blue for 2 weeks, orange for 4 weeks and dark red for 8 weeks closure duration, using four different markers used to distinbuish the two types of school closure intervention and presence/absence of antiviral treatment plus household prophylaxis. We assumed that 50% of symptomatic cases would be diagnosed after 1 day of their symptom's appearance. We further assumed that antiviral treatment and prophylaxis (T+H) began after 10 cases were diagnosed in one day in the community.

Figure 2

Peak daily incidence rate of epidemics with school closure operational issues. Outcomes of two different types of school closure intervention strategies (individual school closure and simultaneous school closure) for 2, 4 and 8 weeks school closure duration with/without antiviral treatment plus household prophylaxis (T+H) for three different R₀ values of 1.5, 2.0 and 2.5 as a function of a number of daily diagnosed cases (activation trigger) are shown. The outcomes are reported in peak daily incidence rate per 10,000 of the population size, and assume a diagnosis ratio of 50%.

Figure 3

Impact of school closure on age-specific attack rates. Age specific attack rates (the proportion of each age group experiencing symptomatic infection) are shown for the baseline case (no interventions), 2 weeks school closure and 8 weeks school closure. The unmitigated epidemic has an R₀ of 1.5. School closure is timed optimally according to policy recommendation in Table 4.

Figure 4

Impact of diagnosis ratio on the effectiveness of school closure activation trigger for R₀ = 1.5. Outcomes of the activation trigger that would give the maximum reduction in attack rate (as Optimal trigger given in Table 3 assuming 50% diagnosis ratio) and the activation trigger at a single symptomatic case (Single case trigger) for individual school closure (ISC) and simultaneous school closure (SSC) strategies in relation to diagnosis ratio (% of symptomatic cases) are shown. The outcomes are reported in percentage of the simulated population size for the epidemics with R₀ values of 1.5 regarding school closure durations of 2, 4 and 8 weeks.

Figure 5

Daily epidemic progression curves for different school closure activation triggers. The daily incidence curves of the simulated epidemics (with R₀ values of 1.5, 2.0 and 2.5) with the activation trigger that would be given the maximum reduction in attack rate (as Optimal trigger) and the activation trigger at a single symptomatic case (as Single case trigger) for individual school closure (ISC) and simultaneous school closure (SSC) strategies for 2, 4 and 8 weeks duration are shown. The red epidemic curves are for the baseline or un-mitigated epidemics for corresponding R₀s. Blue and light green curves are for the prompt triggered ISC and SSC strategies. The other pink and dark green lines are for the best triggered ISC and SSC strategies.