Whether you cast your gaze towards Egypt (H5N1), or China (H7N9), recent outbreaks of avian flu among humans continue to raise concerns over their pandemic potential. Neither virus has demonstrated an ability to spread efficiently from human-to-human, but both continue to get fresh opportunities to try to figure us out.
Last month, in WHO: H5 Currently The Most Obvious Avian Flu Threat, the World Health Organization – while not ignoring H7N9’s potential - weighed in on the risks posed by the recent and dramatic surge in both the variety, and spread, of HPAI H5 viruses around the globe.
A couple of weeks later, in Nature: Dissemination, Divergence & Establishment of H7N9 In China, we saw renewed warnings over the evolutionary path that the H7N9 virus has been taking over the past two years. A topic we revisited the next day, in H7N9: Primus Inter Pares?
To this short list, we must also add a growing variety of reassortant avian and swine viruses (H5N8, H5N6, H5N2, H10N8, H3N8, H3N2v, etc.), many of which have at least some potential to adapt to humans.
Today the EID Journal has published a new review of the transmissibility of the avian H7N9 virus in China, and while no evidence of sustained transmission was detected, they found:
- `evidence of a small but significant amount of transmission between humans in the first and second waves’
- `evidence of increased transmission potential in the second wave’
First a few excerpts from the study (follow the link to read it in its entirety), after which I’ll be back with a bit more.
Volume 21, Number 5—May 2015
To determine transmission potential of influenza A(H7N9) virus, we used symptom onset data to compare 2 waves of infection in China during 2013–2014. We found evidence of increased transmission potential in the second wave and showed that live bird market closure was significantly less effective in Guangdong than in other regions.
From February 19, 2013, through April 22, 2014, a total of 429 cases of influenza A(H7N9) virus infection in humans in China were reported and occurred in 2 outbreak waves. During the first wave in spring 2013, live bird markets were closed in several parts of China (1,2); these market closures substantially reduced the risk for infection in affected regions (3). During a second wave in autumn 2013 (4), markets were again closed in some provinces (5–7). Analysis of the largest clusters of subtype H7N9 virus infection in 2013 suggested that the basic reproduction number (R0, the average number of secondary cases generated by a typical infectious host in a fully susceptible population) was higher in some clusters than in others (8,9), although the absence of sustained transmission implied that R0 was less than the critical value of 1. To determine the transmission potential of influenza A(H7N9) virus in the first and second waves in 2013, we compared symptom onset data. We also measured the extent to which market closures in 2014 reduced spillover hazard (i.e., risk for animal-to-human infection).
<SNIP Study Details>
We found no evidence of reduced human-to-human transmission between the 2 waves. For a serial interval of 7 days, we estimated that R0 increased in Zhejiang. Furthermore, the effectiveness of live bird market closures varied between regions; short-term closures were substantially less effective than interventions in other regions. These results emphasize the value of prompt and sustainable control measures during outbreaks of influenza A(H7N9) virus infection.
Last summer, in Eurosurveillance: Genetic Tuning Of Avian H7N9 During Interspecies Transmission, we saw evidence of the genetic diversity, and continual evolution, of the H7N9 virus in Mainland China. Researchers found that at least 26 separate genotypes had emerged, mostly during the first wave, through a process they called `genetic tuning’.
The Nature report, mentioned above, expands that H7N9 universe to 48 genotypes, spread across three major clades.
Not only are the incarnations of H7N9 continuing to grow, the H7N9 virus has also reassorted into at least two new subtypes on the Chinese mainland; H7N7 (see Nature: Genesis Of The H7N9 Virus) and a new H7N6 virus described in the recent Nature report.
This malleable H7N9 virus, which is spreading asymptomatically and stealthily in China’s poultry, could also potentially reassort with a human influenza virus like H3N2 or H1N1. Something that Hong Kong’s CHP Director has discussed repeatedly this winter (see HK’s Dr. Ko Wing-man On Flu Reassortment Concerns).
While there are seemingly a lot of ways for H7N9 to become a pandemic threat, it is also possible that there is some – as yet unidentified - `species barrier’ that prevents avian flu viruses from adapting well enough to humans to pose a serious threat.
The progression of human influenza pandemics over the past 130 years has been H2, H3, H1, H2, H3, H1, H1 . . . . and while that doesn’t prove that an H5 or an H7 virus couldn’t adapt to humans (or hasn’t in the past), it has led some researchers to wonder whether a non H1, H2, or H3 virus has the `right stuff’ to spark a pandemic.
While some scientists believe that may be a possibility (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?), few are willing to bet the farm on our being that lucky.