Analysis of the effectiveness of interventions used during the 2009 A/H1N1 influenza pandemic

Abstract

Background: Following the emergence of the A/H1N1 2009 influenza pandemic, public health interventions were activated to lessen its potential impact. Computer modelling and simulation can be used to determine the potential effectiveness of the social distancing and antiviral drug therapy interventions that were used at the early stages of the pandemic, providing guidance to public health policy makers as to intervention strategies in future pandemics involving a highly pathogenic influenza strain.

Methods: An individual-based model of a real community with a population of approximately 30,000 was used to determine the impact of alternative interventions strategies, including those used in the initial stages of the 2009 pandemic. Different interventions, namely school closure and antiviral strategies, were simulated in isolation and in combination to form different plausible scenarios. We simulated epidemics with reproduction numbers R0 of 1.5, which aligns with estimates in the range 1.4-1.6 determined from the initial outbreak in Mexico.

Results: School closure of 1 week was determined to have minimal effect on reducing overall illness attack rate. Antiviral drug treatment of 50% of symptomatic cases reduced the attack rate by 6.5%, from an unmitigated rate of 32.5% to 26%. Treatment of diagnosed individuals combined with additional household prophylaxis reduced the final attack rate to 19%. Further extension of prophylaxis to close contacts (in schools and workplaces) further reduced the overall attack rate to 13% and reduced the peak daily illness rate from 120 to 22 per 10,000 individuals. We determined the size of antiviral stockpile required; the ratio of the required number of antiviral courses to population was 13% for the treatment-only strategy, 25% for treatment and household prophylaxis and 40% for treatment, household and extended prophylaxis. Additional simulations suggest that coupling school closure with the antiviral strategies further reduces epidemic impact.

Conclusions: These results suggest that the aggressive use of antiviral drugs together with extended school closure may substantially slow the rate of influenza epidemic development. These strategies are more rigorous than those actually used during the early stages of the relatively mild 2009 pandemic, and are appropriate for future pandemics that have high morbidity and mortality rates.

Figure 1

Dynamics of influenza progression within host individuals. (a) Dynamics of influenza A/H1N12009 progression within host individuals. (b) Infectivity curve of an infectious individual due to influenza infection.

Figure 2

Epidemic progression curve expressed as daily incidence rate and cumulative illness rate. Top pair shows comparison among different school closure strategies with 1 week closing period. Central pair indicates the potential impact of antiviral drug usage during simulated pandemic period. Bottom pair illustrates the combined effectiveness of individual school closure for 1 week and antiviral strategies during pandemic. Results are illustrated in terms of daily incidence rate (symptomatic cases per day, left column) and cumulative symptomatic attack rate (percentage of population, right column).

Figure 3

Final attack rate of epidemics with concurrent school closure and antiviral based strategies. Final attack rate (% of population) is presented for an epidemic with R₀ = 1.5 and different antiviral based strategies in conjunction with individual school closure for variable school closure durations (0 week to 4 weeks).

Figure 4

Peak daily incidence rate of epidemic with concurrent school closure and antiviral based strategies. The figure illustrates the impact of different durations of school closure coupled with antiviral based strategies on peak daily incidence rate during an epidemic. Peak daily incidence rate is expressed in symptomatic cases per 10000 population.

Figure 5

Required antiviral courses (% of population) for an epidemic with R₀ = 1.5. The figure illustrates the consumption of antiviral drugs for different antiviral strategies combined with school closures during a pandemic.