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[Estimating the incidence of 2009 pandemic influenza A(H1N1) among IMSS affiliates].

INTRODUCTION
The purpose of this study is to estimate the burden of the disease associated to pandemic 2009 influenza virus, from April 2009 to January 2010.


METHODS
To estimate the number of symptomatic cases, the number of hospitalizations and deaths we used the Center for Disease Control (CDC) recommended method that takes into account the underestimation associated with the use of health services, the practices of confirmation and registration of cases.To estimate the incidence of infection, we applied the recently reported London sero-incidence by age group to the IMSS population.


RESULTS
Each case of symptomatic confirmed influenza represented 51 cases during the first wave and 18 in the second wave. We estimate 537,167 (range 378,439-813,008) symptomatic cases. Each confirmed hospitalized person represented 2.2 cases. The estimate of hospitalizations was 10,063 (range 7,441-14,610). The ratio of hospitalization to the total number of cases was 1.8%. The estimated incidence of infection was close to 24%.


CONCLUSIONS
Confirmed cases in the epidemiological surveillance system are only a small proportion of the population infected and symptomatic cases, information relevant in planning new outbreaks.