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.

Infection with a novel swine-origin influenza A (H1N1) virus, currently named pandemic (H1N1) 2009 virus, first occurred in the United States and Mexico in early April 2009 (

To estimate the prevalence of pandemic (H1N1) 2009 in the United States, the US Centers for Disease Control and Prevention (CDC) developed a software program (Impact2009, version 1.0) (

From the virologic surveillance data, we determined that positive cases of pandemic (H1N1) 2009 were identified through August 3, 2009. From this finding, we used 2 phases for the model: phase 1 (May 16, 2009, through August 2, 2009) and phase 2 (August 3, 2009 through December 31, 2009). In addition, the consultation rate for ILI cases had changed over the course of the pandemic, because of changes in strategies used to control the disease in Beijing before and after National Day (October 1). To adjust for the introduction of these strategies, we further divided phase 2 into 2 periods: period 2a (from August 3, 2009, through September 30, 2009) and period 2b (from October 1, 2009, through December 31, 2009).

During phase 1, the number of laboratory-confirmed cases was considered to reflect the true number of pandemic (H1N1) 2009 infections. However, during phase 2, we calculated the true number of infections by multiplying the baseline by the estimation coefficient, using the multiplier model. In this multiplier model, the baseline case number was equal to the sum of the product of the weekly ILI case number in level 2 and 3 hospitals and the corresponding weekly pandemic (H1N1) 2009 positive rate among case-patients with ILIs. The estimation coefficient was found by multiplying the reciprocal of the parameters in the model. The following parameters were required in our estimation: the proportion of symptomatic infection among patients with cases of pandemic (H1N1) 2009, the proportion of ILI among patients with symptomatic cases of pandemic (H1N1) 2009, the consultation rate among ILI case-patients, the sampling success rate, and the sensitivity of the test (

Model parameters for estimating the true number of persons infected with pandemic (H1N1) 2009 in Beijing. A, hospitals refer to level 2 and 3 hospitals in Beijing; B, sampling success rate was included in the model because not all actual positive specimens gave positive results because of the timing of collection or the quality of the specimen; C, test sensitivity was included in the model because not all actual positive specimens gave positive results due to the insensitivity of PCR reagent and unpredictable errors in experimental operations and instruments; D, proportion of true pandemic (H1N1) 2009 cases for which specimens were successfully collected; E, proportion of true positive specimens that were correctly identified by PCR reagent. ILI, influenza-like illness.

Code | Parameter | Value, % | Source |
---|---|---|---|

A | Proportion of symptomatic infection among case-patients with pandemic (H1N1) 2009 | 70–75 | Pandemic (H1N1) 2009, ECDC Risk Assessment.,2009; version 6, 6 Nov. |

B | Proportion of ILI among symptomatic case-patients with pandemic (H1N1) 2009 | 26–42 | Literature and unpublished clinical data |

C1 (period 2a) | Consultation rate among ILI case-patients in secondary and tertiary hospitals | 38 | Telephone interview conducted by Beijing CDC |

C2 (period 2b) | Consultation rate among ILI case-patients in secondary and tertiary hospitals | 48 | Telephone interview conducted by Beijing CDC |

D | Sampling success rate | 80–90 | Previous surveillance data |

E | Sensitivity of test | 95–100 | Professional recommendations |

*ECDC, European Centre for Disease Prevention and Control; ILI, influenza-like illness; Beijing CDC, Beijing Center for Disease Prevention and Control. †The multiplier model was only used for phase 2 in this study, and phase 2 was divided into 2 periods, period 2a and period 2b. During phase 2, the true number of infections was calculated by multiplying the baseline by the estimation coefficient, using the multiplier model. The baseline case number was equal to the sum of product of weekly ILIs number in level 2/3 hospitals and the corresponding weekly pandemic (H1N1) 2009 positive rate among case-patients with ILIs. The estimation coefficient was obtained by multiplying the reciprocal of the parameters mentioned in this table. The baseline case numbers in periods 2a and 2b were 6,520 and 171,899, respectively.

In phase 1, a total of 325 positive cases were reported (considered as the true infection number). In period 2a and period 2b of phase 2, the baseline case numbers were 6,520 and 171,899, respectively. During phase 2, a total of 1,800,074 pandemic (H1N1) 2009 infections were estimated. Thus, by the end of 2009, the cumulative number of persons infected with pandemic (H1N1) 2009 in Beijing was estimated to be 1,800,399 (90% range 1.46–2.30 million) (

Age group, y | Proportion of total no. persons infected, % | Estimated no. cases, median (90% CI) | Estimated rate, %, median (90% CI) |
---|---|---|---|

0–4 | 13.4 | 241,253 (195,910–307,571) | 30.8 (25.0–39.2) |

5–14 | 35.1 | 632,300 (513,459–806,111) | 31.8 (25.8–40.6) |

15–24 | 29.4 | 528,597 (429,247–673,902) | 22.2 (18.0–28.3) |

25–59 | 20.9 | 375,383 (304,829–478,571) | 4.1 (3.3–5.2) |

1.3 | 22,865 (18,568–29,150) | 0.9 (0.7–1.1) | |

Total | 100.0 | 1,800,399 (1,462,012–2,295,305) | 10.6 (8.6–13.5) |

*CI, confidence interval.

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 (

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% (

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.

Although, in theory, serologic surveys should provide an accurate record of the infection rate of pandemic (H1N1) 2009, they failed to provide a quicker and more representative result than the multiplier model. Given the similarities between the estimates obtained from the model and the estimates obtained from the serologic survey, we conclude that the multiplier model based on the Monte Carlo approach should be considered a useful and simple method for estimating the true number of infections during a pandemic.

This study was funded by Beijing Natural Science Foundation (7082047), National 863 Project (2008AA02Z416), National Sci-Tech Key Projects During the Eleventh Five-Year Plan Period (2009ZX10004-315), and Key Task Of Novel H1N1 Flu Prevention Strategy of Beijing Sci-Tech Bureau (Z09050700940905).

Dr Xiaoli Wang is a medical epidemiologist at the Institute for Infectious Disease and Endemic Disease Control of Beijing Center for Disease Prevention and Control. Her research interests are the epidemiology of respiratory infectious diseases and early-warning surveillance systems for emerging infections.