Estimates of the True Number of Cases of Pandemic (H1N1) 2009, Beijing, China

DHHS/CDC Emerging Infectious Diseases
Volume 16, Number 11–November 2010
Xiaoli Wang, Peng Yang, Holly Seale, Yi Zhang, Ying Deng, Xinghuo Pang, Xiong He, and Quanyi Wang Comments to Author
Author affiliations: Beijing Center for Disease Prevention and Control, Beijing, People's Republic of China (X. Wang, P. Yang, Y. Zhang, Y. Deng, X. Pang, X. He, Q. Wang); Capital Medical University School of Public Health and Family Medicine. Beijing (X. Wang, P. Yang, Y. Zhang, Y. Deng, X. Pang, X. He, Q. Want); and University of New South Wales, Sydney, New South Wales, Australia (H. Seale)

Abstract
During 2009, a total of 10,844 laboratory-confirmed cases of pandemic (H1N1) 2009 were reported in Beijing, People's Republic of China. However, because most cases were not confirmed through laboratory testing, the true number is unknown. Using a multiplier model, we estimated that ≈1.46–2.30 million pandemic (H1N1) 2009 infections occurred.
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Conclusion

Despite the small number of laboratory-confirmed cases (10,844), we estimated that the actual number of persons infected with pandemic (H1N1) 2009 was 1.8 million in Beijing by the end of 2009. Previous studies have claimed that the number of laboratory-confirmed cases of pandemic (H1N1) 2009 was substantially underestimated, reflecting only a very small fraction of the actual infections (3–5). This study also demonstrated that school age children were more likely to be infected with pandemic (H1N1) 2009. However, those >60 years of age were at low risk for infection.

From November 27 through December 7, 2009, a serologic survey to establish the prevalence of pandemic (H1N1) 2009 antibody was conducted in the general population of Beijing. The results showed that ≈14%–15% (13) of the general population had antibodies to pandemic (H1N1) 2009 virus. Based on the population size of 17 million in Beijing in 2009 (14) and the assumption that antibodies against pandemic (H1N1) 2009 virus are usually produced after 2 weeks of infection or vaccination (15), we estimated that 2.37 to 2.54 million persons were infected with pandemic (H1N1) 2009 virus as of November 13, 2009. According to data from the Beijing Center for Disease Prevention and Control, by November 13, 2009, 1.36 million persons had received the pandemic (H1N1) 2009 vaccine. After the vaccinated population were removed from the equation, the total number of pandemic (H1N1) 2009 cases was estimated to be ≈1.01 to1.18 million. At the same time, the number of infections was estimated at 0.87–1.28 million as of November 13, 2009, by the multiplier model (data not shown in the section of the study). The estimates of the infection matched with the actual number estimated from the serologic survey in principle.

In phase 1, the number of laboratory-confirmed cases was considered to reflect the true infection number. This assumption, however, may lead to an underestimation for 2 reasons. First, we ignored the parameters used in phase 2, and second, difficulties occurred in testing all of the samples taken from patients who sought consultation for ILIs. Nevertheless, because the pandemic did not spread in the community in phase 1, we believe that this underestimation would have been quite low.