Introduction: The first case of pandemic influenza A(H1N1) was detected in Singapore on 26 May 2009, 1 month after the first cases of novel influenza A(H1N1) was reported in California and Texas in the United States. The World Health Organization declared the first influenza pandemic of the 21st century on 11 June 2009.Materials and Methods: Confirmed cases notified to the Ministry of Health between 27 May and 9 July 2009 were analysed. Various indicators of influenza activity were monitored throughout the study period. Estimates of the number of cases of H1N1-2009 were made using the number of polyclinic attendances for acute respiratory infection and influenza-like illness and the weekly prevalence of H1N1-2009. Results: Cases in Singapore affected mainly young adults, youths and children. By the end of September 2009, it was estimated that at least 270,000 persons had been infected by pandemic influenza A (H1N1) in Singapore. The peak number of cases occurred during E-week 30 (26 July-1 August) when an estimated 45,000 cases were seen in polyclinics and GP clinics. The hospitalisation, severe illness and mortality rates were estimated at 6 per 1000 cases, 0.3 per 1000 cases and 6.7 per 100,000 cases, respectively. The most common risk factors among hospitalised adult cases were asthma and diabetes. For hospitalised children, the most common risk factors were being under 5 years of age and asthma. The most common risk factors among persons with severe illness were diabetes in adults and epilepsy and being under 5 years of age in children. About half of cases with severe illness required mechanical ventilation. In addition, one-fifth of cases with severe illness had acute respiratory distress syndrome. Conclusion: The first wave of the influenza pandemic lasted about 10 weeks. Morbidity and mortality resulting from pandemic influenza were low.
The first cases of infection with a novel influenza A (H1N1) strain were reported in 6 cases in California and 2 cases in Texas in the United States in late April 2009. This was linked to outbreaks of influenza in Mexico, which included reports of large numbers of cases of severe pneumonia.
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