The first officially reported human case of infection from a reassortant avian influenza A(H7N9) virus was from the People’s Republic of China (China) in March of 2013, although human H7N9 infection may have occurred in or near Hong Kong as early as 2007 (FAO ID event 220957). Since 2013 the World Health Organization (WHO) has officially reported 808 human cases of H7N9 as of December 23, 2016. In the past few days, an additional 13 H7N9 cases have been reported by public health officials in China but have not yet been published by the WHO. Of these 821 cases, 696 have onset or reporting dates prior to December 31, 2015. The total number of reported H7N9 cases in 2016 is 125.
Geographic Distribution
All 125 human cases of H7N9 in 2016 were reported from
China. These cases have been reported from 18 provinces and special
administrative regions. More specific geographic location information was
available for 117 of these cases, the remaining 8 cases were only reported from
a specific province; Anhui 2, Hebei 2, Fujian 1, Hubei 1, Shandong 1, and
Zhejiang 1. The map below depicts the provinces and special administrative
regions with reported 2016 cases. Dots indicate the locations of individual or
multiple cases reported in 2016. In 2016, these cases all occurred in the
Western portion of China.
Table: Comparison of the frequencies of human H7N9 infections reported in China between 2013-2015 and 2016 by province/special administrative regions.
Map: Geographic distribution of human H7N9 cases in China in 2016.
Age Statistics
From 2013 to 2015, the median age of H7N9 infected males was
57.5 years with ages ranging from 1 to 91 years old. For females during that
period the median age was 54.5 with ages ranging from less than year to 85
years old. The age distributions by gender for 2016 is similar. The chart below
compares age categories for 2013-2015 and 2016. In 2016, infected individuals
were somewhat younger than previous years.
Graph: Comparison of the frequencies of human H7N9 cases by age categories.
Gender
Between 2013 and 2015, 471 of the reported cases were male (68%)
and 220 of the reported cases were female (32%). The genders of the remaining
cases were not published. Among the 125 cases reported in 2016, 72% (85) were
male and 28% (33) were female. Seven cases do not have a reported gender.
It seems that males are far more likely to contract H7N9
infections than females. Because almost all cases are associated with exposure to
infected poultry, it is possible that different gender roles expose males more
frequently to affected poultry than females.
Fatalities
Between 2013 and 2015, 143 of the 696 reported H7N9 cases were
reported as fatal. These data would suggest a minimum case fatality risk (CFR)
of .21.
The CFR, as defined here, is the conditional probability of
death from an H7N9 infection, a ratio between H7N9-caused deaths and
recoveries/asymptomatic cases. Because follow-up reporting is lacking in many
of these cases, the actual number of deaths versus the number of recoveries is
uncertain. Very few of the cases in the period from 2013 to 2015 were officially
reported as recovered (only 133).
In 2016, 29 of the 125 cases were reported as fatal. At face
value, the CFR for 2016 is .23 but it is likely to be higher since many of the
recently reported cases are currently being treated.
H7N9 Clusters
Most of the reported human H7N9 infections in 2016 result
from zoonotic transmission of the virus from domestic poultry. Public health
reports in 2016 rarely indicate the possibility of human-to-human transmission
among confirmed H7N9 cases which would signal a cluster of cases. A human
cluster of cases is generally defined by WHO as two or more cases of confirmed,
probable, or suspected infections with onset of illness occurring within the
same two-week period and who are in the same geographical area and/or are
epidemiologically link.
Based on family ties or restricted geographic area, potential
human H7N9 clusters in 2016 include the following. In February 2016 infected
individuals in several locations may represent multiple clusters, two siblings
in Suzhou Jiangsu, several family members in Wuxi Jiangsu, and several
unrelated individuals in Fuzhou Fujian. Several family members from Tahie Jiangxi
were reported as H7N9 cases in April. Two cases from Hebei in July of 2016 may
represent a cluster. Recently, two infected individuals from Kunshan Jiangsu were
reported in November and three cases in Hefei Anhui in December, with no other
details are available.
Discussion
Most of the human H7N9 case reported in 2016 are sporadic
infections. As noted above, a few clusters of cases suggest that human-to-human
transmission may have occurred but did not result in sustained human-to-human
transmission. H7N9 cases seems to occur on an annual cyclical basis that
follows the pattern of season human influenza infections. As depicted in the
chart below, H7N9 cases were frequent from week numbers 1-23 and started
increasing again in week number 46. The seasonal fluctuation in human infections
indicates that more H7N9 cases can be expected in the coming months. While the
number of H7N9 cases declined in 2016 compared to earlier years, the potential for
a deadly epidemic or a possible H7N9 pandemic continues to exist.
Chart: Epidemic curve of
human H7N9 cases in 2016.
Note: The information presented and discussed here is based
on a compilation of publicly available data sources including WHO, Food and
Agriculture Organization of the United Nations, and various public health
agencies supplemented by media reports as available.
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