Since November 2016, more than 460 human cases of H7N9 have been reported or imported from China. To put this number in perspective, confirmed cases of H7N9 were first reported in March 2013, four years ago. Of all the cases of human H7N9 infections reported to date, more than one-third (about 36%), have occurred in the last four months. This raises a concern that H7N9 is not only causing outbreaks in China but could lead to epidemics and perhaps even a pandemic.
It is difficult to tabulate exactly how many H7N9 cases have occurred since November 1 of 2016, because case reporting and enumeration seem to vary among various public health reporting agencies. Media and blog reports have interpreted variation among these counts of H7N9 cases as a failure of public health officials in China to accurately track H7N9 cases, often leading to exaggerated claims of the rates of infection in China.
It is possible to arrive at a close approximation of the actual number of recent cases by using different data sources. Official counts of human cases of H7N9 are individually presented by the Food and Agricultural Organization of the United Nations (FAO), the Centre for Health Protection (CHP), and the World Health Organization (WHO). The discrepancies between these different agencies can be attributed to differential reporting periods. The FAO updates its list of human H7N9 infections every few days or whenever newly confirmed cases are reported. The CHP only updates its case information every seven days in the Weekly Influenza Report. The WHO only provides irregular updates, often only in aggregate fashion in the Disease Outbreak News. Later the WHO usually provides case details in its Influenza at the Human Animal Interface: Summary and Assessment that is only published on a monthly basis.
In order to compare these three data sets we need to have a starting point. The current outbreak in China started in November 2016. Prior to that time only a few sporadic cases were reported in the preceding weeks. Between November 1, 2016 and February 16, 2017 (the last date of FAO reported cases), the FAO has noted 437 cases of human H7N9. On November 1, 2016, the WHO count of human H7N9 cases was 800. The most recent WHO Disease Outbreak News H7N9 reports a total of 1223 confirmed H7N9 cases, indicating a total of 423 cases since November 1, 2016. The WHO case counts however only includes cases reported through February 14, 2017.
Prior to November 1, 2016, the CHP reported a total of 798 cases. Since then, the CHP has reported 461 H7N9 cases through February 27, 2017. Adjusting the FAO number of cases to include 35 cases noted by CHP with reporting dates after February 16, would bring the total FAO case count through February 27 to 472 for the period from November 1, 2016 – February 27, 2017. Adding the 43 additional cases noted by CHP (and not yet reported by WHO) to the WHO-reported count of 421 gives a total of 465 confirmed cases for the period of November 1, 2016- February 27, 2017. The variations between the adjusted counts of these three agencies is minimal, the average is 466 cases. Based on these data, the WHO count of H7N9 cases through February 27 should eventually be reported to be about 1267 cases.
The differences between the H7N9 case counts among these three agencies are primarily a function of differential reporting dates, and not the result of confusion about the number of cases by Chinese public health officials.
Another question to ask is how accurate is this count of H7N9 cases from China. In the past, China has been accused of underreporting infectious diseases to the WHO. Could there be hundreds of more human H7N9 cases that are not being reported to the WHO?
We can compare the sex ratio and median age of the 461 cases noted by the CHP in this outbreak with the sex ratio and median age reported for the previous 792 confirmed H7N9 cases prior to November 1, 2016. For the cases prior to November 1, 2016, males represent 68% of the cases, and females represent 32% of the cases. In the current outbreak, males represent 71% of the cases and females represent 29% cases. These numbers are within the range of statistical variation.
For the cases prior to November 1, 2016, males had a median age of 58 and females had a median age of 55. For the cases in the current outbreak, males have a median age of 58 and females a median age of 56. The median age for both males and females is comparable from the cases in this outbreak to all of the previous H7N9 cases reported.
Thus, there is no reason to assume that China has been underreporting H7N9 cases during this outbreak. Hypothetically, in order for China to be underreporting current H7N9 cases, the public health authorities would have to be implementing a sophisticated real-time algorithm that would allow cases to be underreported, yet still maintain the male to female ratio and the average median age for the remaining cases. Because China is reporting new H7N9 cases every few days, it does not seem possible for China to be purposefully underreporting cases of H7N9 in this outbreak. Internet claims of hundreds of unreported human H7N9 cases in China are unfounded.