Objective: To estimate the real number of novel influenza A (H1N1) infection in Beijing, 2009.
Methods: A multiplier model (Impact 2009 v 1.0 software) based on Monte Carlo approach was used to estimate the real number of novel influenza A (H1N1) based on the number of influenza-like illness (ILI) cases, novel influenza A (H1N1) positive rate among ILI cases and rate on clinical visit of ILIs in secondary and tertiary hospitals.
Results: There were 1.80 million (90%CI: 1.46 - 2.30) estimated novel influenza A (H1N1) cases in 2009 in Beijing with the rate of infection as 11.0%. One reported case would represent 167 real infections. The highest age groups of infection were 0 - 4 years and 5 - 14 years, being 32.5% and 33.3%, respectively.
Conclusion: Laboratory-confirmed infections with novel influenza A (H1N1) only represented a fraction of the total cases in a population, suggesting that it was imperative to estimate the real number of novel influenza A (H1N1) infection.