Based on illness onset dates from January through the end of November 2016, China officially reported about 115 human cases H7N9 infection. Over the course of several days in early January 2017, China notified the World Health Organization of more than 100 additional human cases of H7N9 presumably having been infected in December 2016. It appears that almost as many people were infected in December as all of the preceding months in 2016.
The graph below shows the distribution of H7N9 cases by onset date where available and then by reporting date. The graph clearly shows the large increases in the number of infected individual reported recently. Should this increase be a cause for alarm?
Increases in human cases of avian influenza always increase the risk for sustained human to human transmission of the disease. Reviewing the minimal data that is available for the 107 recent cases reported by China, some observations can be made. About 36% of these new cases are female and 67% are male. This gender ratio is similar to the earlier cases in 2016. The age range of these cases is 23 to 91 years with a median age of 54 years old, also similar to the age distribution of earlier cases in 2016. There is no evidence from these recent cases that different age groups are being disproportionately infected.
Finally, the case fatality risk (CFR) for these recent cases is about .31. This is higher than the CFR for earlier cases in 2016 and the overall CFR for all cases since the initial outbreak in 2013. Many of the nonfatal cases are reported to have severe pneumonia, which suggests that more of these individuals may not recover.
There is nothing in the publicly available reports of these cases which would indicate the extent of human to human transmission, if any. The best indirect way to assess the potential for human to human transmission is to evaluate human clusters. In the available data, there is no information about relationships among various infected individuals, nor are onset dates available to assess chains of transmission.
The only information we currently have available to interpret potential clusters is the geographic distribution of cases. The map below plots the geographic distribution of human cases recently reported by China compared with all of the H7N9 cases with onset dates or reported dates since January 1, 2016. About half of these newly reported cases are spread out among various provinces in eastern China and probably represented isolated sporadic infections.
However, the remaining 50+ cases were reported from just four cities. The map below shows the four cities with 9 or more H7N9 cases reported in the January announcements, Suzhou, 21 cases, Wuxi 11 cases, and Changzhou 10 cases, all in Jiangsu Province. The fourth city is Hangzhou in Zhejiang Province with 9 cases. All of these cities are large population centers, so we will need more case details to determine if there is human to human transmission in these areas.
Information on contact tracing would be useful as well. None of the reported cases appears to be asymptomatic. Less 10 cases since the initial human H7N9 outbreak have been reported as asymptomatic. Are mild cases being overlooked?
If the number of reported H7N9 cases continues to grow dramatically over the next several weeks, it may signal a local H7N9 epidemic in China. We need to be watching H7N9 in China very closely.