We used initial reporting data on laboratory-confirmed cases of pandemic H1N1 influenza in the province of Ontario to estimate key epidemiologic parameters for pandemic H1N1 influenza. Because the 2009 influenza pandemic continues to evolve, these values are critical for planning and can be used to reduce some of the uncertainty around the health burden likely to be associated with this disease in the coming months. Our low estimate of 1.31 for the basic reproductive number (R0
) is consistent with estimates from Mexico4,5
(1.2 to 1.6) and the United States (1.7–1.8 after adjustment for increasing ascertainment of cases).6
estimate is also within a range where mitigation strategies, including use of antiviral drugs, social distancing and limited vaccination, might substantially reduce the final size of the epidemic.7–11
Our estimated R0
may be lower than the true value for pandemic H1N1 influenza. Public concern related to the emerging pandemic may have changed population behaviour or medical practices, thereby reducing observed reproductive numbers. Furthermore, a striking feature of the current pandemic has been a concentration of cases in younger groups, which suggests the presence of pre-existing immunity in older adults.12,13
The effective reproductive number in a partially immune population is lower than the basic reproductive number.14
Despite reduced susceptibility, we have estimated that older people with pandemic H1N1 influenza are at increased risk of hospital admission and death, which emphasizes the need to characterize the relation between age and risk of infection early in a pandemic.15
The discrepancy between age groups in susceptibility to infection and severity of illness may be an important factor limiting the burden of hospital admision and deaths in the current pandemic.
Our estimate of mean incubation period (time from infection to onset of symptoms) of 4.3 days is compatible with our model’s upper bound for latent period (time from infection to infectiousness) of 3.1 days, assuming a 1-day period of infectiousness before onset of symptoms. Both of these estimates are longer than is typical for seasonal influenza.16
By contrast, our estimate of the duration of symptoms (median 7 days) is longer than our model-based estimate (mean 3.4 days). This difference could reflect effective isolation of cases or persistent symptoms that outlast infectiousness. However, our estimate of symptom duration is consistent with emerging data on shedding of the pandemic H1N1 influenza virus.17
The long latent and infectious periods we have described resulted in longer estimated serial intervals for this disease than those previously generated by Fraser and colleagues using indirect methods.4
The laboratory-confirmed cases we analyzed almost certainly represent a small subset of cases of pandemic H1N1 influenza during the period under study. We did not include infected people without symptoms or those with symptoms who did not undergo laboratory testing. Knowledge of the proportion of infected people who are actually identified as cases is needed to estimate the true risk of hospital admission or death among cases.18
Estimates derived early in the current pandemic suggested that only 10% of people in developed countries received laboratory confirmation of pandemic H1N1 influenza.18
More recent estimates are as low as 1.25%.19
Thus, it may be reasonable to reduce our estimates of hospital admission and case-fatality rates by a factor of 10 or more in order to approximate risks among all people with pandemic H1N1 influenza. Serologic studies will be essential in developing more refined estimates of the true proportion of the population infected during the period under study.
Our study has several limitations. First, our projection of hospital admissions and deaths among laboratory-confirmed cases, and with a limited number of outcomes, introduced wide confidence intervals in the risk estimates. As the pandemic progresses, we will be able to refine these values. However, our initial estimates provide some upper and lower bounds that may be useful for planning, if it is understood that laboratory-confirmed cases represent only a subset of all cases of pandemic H1N1 influenza in the community. Second, our estimates of the duration of symptoms may have been biased downward. Although we attempted to account for missing dates of symptom resolution by using statistical methods, some of the outcomes may have been censored because the patients had prolonged illness. Finally, not all hospital admissions or deaths attributable to influenza will be captured in surveillance data. For seasonal influenza, about 30% of related deaths have been predicted to occur outside of hospital,20
and the burden of influenza difficult to assess.21,22
On the basis of data from initial laboratory-confirmed cases of pandemic H1N1 influenza, our estimates show that the characteristics of this novel influenza virus do not differ substantially from those of seasonal influenza. However, when combined with high attack rates in younger groups,13
there may be greater absolute numbers of hospital admissions and deaths than are observed in a typical influenza season. Enhanced surveillance during an influenza pandemic, compared with surveillance during seasonal influenza epidemics, could also contribute to such an effect. Our relatively low estimate of R0
(the number of new cases created by a single primary case in a susceptible population) indicates that effective use of mitigation strategies may substantially reduce the final size of the pandemic.