|Credit EID Journal|
Despite having the ability to map the genetic code of influenza viruses down to the molecular level, the observation of how viruses behave in their hosts, and their patterns of transmission, remain our most important tools for understanding influenza's evolution and threat.
In the winter-spring of 2013 a new, LPAI (low path) H7N9 virus appeared in China, and while it produced little or no morbidity in birds, it was (to everyone's surprise) highly pathogenic in humans.
On top of that, it was an H7 virus - and up until that time - H7 viruses were viewed as minor threats to human health. Suddenly the venerable H5N1 virus - which had killed millions of birds and infected roughly 600 people over the previous decade - had a surprisingly stealthy and deadly competitor.
Since H7N9 emerged three years ago, we've watched as it has continued to evolve (see EID Journal: H7N9’s Evolution During China’s Third Wave – Guangdong Province).
The H7N9 virus had, as of early 2015 (see Nature report), produced at least 48 genotypes, spread across three major clades, and it is likely that this constellation of H7N9 variants has continued to expand since that time.
Up until 14 months ago, we received real-time detailed, daily updates from China on their H7N9 cases, including location, age, gender, onset date, likely exposure, and condition or outcome.
That all ended abruptly early in 2015, and since then we've gotten mostly belated reports - sometimes weeks or months after the fact - often with little or no patient details attached (see China Reports 11 H7N9 Cases (7 Fatalities) In April). This dearth of data has made tracking H7N9's progress in China increasingly difficult.
Today we have an EID Journal report that sheds some light on the changing patterns of H7N9 infection in China, and fills in some of the missing data.
The authors find that patients hospitalized in the 2nd and 3rd wave with severe H7N9 tended to be younger, and from more rural areas, than those from the 1st wave. They also found that the risk of death among hospitalized patients was greater in the second and third waves, although that varied between provinces.
Exactly why the demographics of the later outbreaks have changed is a much tougher question to answer, although the authors suggest:
The increased risk in waves 2 and 3 might imply a changing pathogenesis associated with genetic clades of H7N9 virus that appeared in later epidemic waves or differences in clinical management in different provinces, although case ascertainment bias could not be ruled out.
Follow the link to read the report in its entirety.
Since March 2013, a novel influenza A(H7N9) virus has caused 3 epidemic waves of human infection in mainland China. We analyzed data from patients with laboratory-confirmed influenza A(H7N9) virus infection to estimate the risks for severe outcomes after hospitalization across the 3 waves.
We found that hospitalized patients with confirmed infections in waves 2 and 3 were younger and more likely to be residing in small cities and rural areas than were patients in wave 1; they also had a higher risk for death, after adjustment for age and underlying medical conditions. Risk for death among hospitalized patients during waves 2 and 3 was lower in Jiangxi and Fujian Provinces than in eastern and southern provinces.
The variation in risk for death among hospitalized case-patients in different areas across 3 epidemic waves might be associated with differences in case ascertainment, changes in clinical management, or virus genetic diversity.More than 3 years have passed since novel influenza A(H7N9) virus infections were first detected among humans in mainland China (1). The first epidemic of human infections occurred in the spring of 2013; 134 cases were laboratory confirmed through September 2013 (control measures in combination with environmental factors led to a lull in incidence in the summer of 2013) (2,3).
However, a second epidemic of infections occurred in the winter of 2013–14 (4), and a third epidemic occurred in the winter of 2014–15. A fourth wave is ongoing, and as of March 3, 2016, in mainland China, 730 laboratory-confirmed human cases of influenza A(H7N9) virus infection have been reported, most associated with severe disease; 295 of the infections were fatal. Low pathogenicity of influenza A(H7N9) infections in chickens has been well established (5), and most human infections can be attributed to close contact with infected chickens, particularly in live poultry markets (1,6,7).
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