Estimated epidemiologic parameters and morbidity associated with pandemic H1N1 influenza

Abstract

Background: In the face of an influenza pandemic, accurate estimates of epidemiologic parameters are required to help guide decision-making. We sought to estimate epidemiologic parameters for pandemic H1N1 influenza using data from initial reports of laboratory-confirmed cases.

Methods: We obtained data on laboratory-confirmed cases of pandemic H1N1 influenza reported in the province of Ontario, Canada, with dates of symptom onset between Apr. 13 and June 20, 2009. Incubation periods and duration of symptoms were estimated and fit to parametric distributions. We used competing-risk models to estimate risk of hospital admission and case-fatality rates. We used a Markov Chain Monte Carlo model to simulate disease transmission.

Results: The median incubation period was 4 days and the duration of symptoms was 7 days. Recovery was faster among patients less than 18 years old than among older patients (hazard ratio 1.23, 95% confidence interval 1.06-1.44). The risk of hospital admission was 4.5% (95% CI 3.8%-5.2%) and the case-fatality rate was 0.3% (95% CI 0.1%-0.5%). The risk of hospital admission was highest among patients less than 1 year old and those 65 years or older. Adults more than 50 years old comprised 7% of cases but accounted for 7 of 10 initial deaths (odds ratio 28.6, 95% confidence interval 7.3-111.2). From the simulation models, we estimated the following values (and 95% credible intervals): a mean basic reproductive number (R0, the number of new cases created by a single primary case in a susceptible population) of 1.31 (1.25-1.38), a mean latent period of 2.62 (2.28-3.12) days and a mean duration of infectiousness of 3.38 (2.06-4.69) days. From these values we estimated a serial interval (the average time from onset of infectiousness in a case to the onset of infectiousness in a person infected by that case) of 4-5 days.

Interpretation: The low estimates for R0 indicate that effective mitigation strategies may reduce the final epidemic impact of pandemic H1N1 influenza.

Figure 1

Age distribution of 3152 laboratory-confirmed cases of pandemic H1N1 influenza in the province of Ontario with onset of symptoms between Apr. 13 and June 20, 2009.

Figure 2

Breakdown of laboratory-confirmed cases of pandemic H1N1 influenza available for analysis.

Figure 3

Estimates of the incubation period (time from exposure to onset of symptoms) in 316 laboratory-confirmed cases of pandemic H1N1 influenza for whom these data were available. Estimates were calculated as the time between exposure date (midpoint of earliest and most recent exposure) and date of symptom onset. The blue curve represents the best-fit log-normal distribution of observed data.

Figure 4

Estimates of the duration of symptoms (time from onset to resolution of symptoms) in 712 laboratory-confirmed cases of pandemic H1N1 influenza for whom these data were available. The blue curve represents the best-fit log-normal distribution of observed data.

Figure 5

Estimates of the basic reproductive number (R₀, number of new cases created by a single primary case in a susceptible population) in the province of Ontario during the wave of pandemic H1N1 influenza in the spring of 2009. Estimates were generated with the use of Markov Chain Monte Carlo simulation modelling. The mean estimate for R₀ is 1.31 (95% credible interval 1.25–1.38).