Case-Based Surveillance of Pandemic (H1N1) 2009 in Maebashi City, Japan

Abstract

After national case-based surveillance for pandemic influenza A (H1N1) ceased on July 23, 2009, a daily case-based surveillance system was implemented in Maebashi City, Japan. All medical facilities in the city reported all patients who had positive rapid antigen tests for influenza A. When the epidemic exploded in late October, case-based surveillance for influenza-like illness (ILI) was implemented from November 3, 2009 until the end of the epidemic. A total of 7,781 influenza cases were reported between July 25 and November 2, 2009, with a cumulative incidence rate of 22.5 per 1,000 population. Nearly 70% of the patients were under 15 years old. Between November 3, 2009 and the end of March 2010, a total of 16,394 ILI cases were reported, with a cumulative incidence rate of 47.4 per 1,000 population. Of the ILI cases reported, 63% were in patients younger than 15 years old. Only one death with laboratory confirmation of the H1N1 2009 virus was reported during the epidemic. The age-specific reproduction number among children under 15 years of age was almost 1.40, whereas between children and adults (15 years of age and above) it was considerably less than 1.0. The reproduction number derived from the next-generation matrix using data from September 30 to October 14 was estimated to be 1.48 (95% confidence interval, 1.41-1.56). Among individuals under 15 years of age, the infection rate calculated using the final size equation under the assumption of no mitigation measures was nearly twice the rate reported during the epidemic. These findings indicate that the majority of the transmission of influenza A (H1N1) 2009 in the city occurred among children.