3 in 50 mostly asymptomatic workers handling live poultry have H7N9 antibodies...

Earlier in the week Yang and colleagues, publishing in the Journal of Infectious Diseases, found that among 1570 people from Zhejiang province tested for antibodies towards influenza A(H7N9) virus, 25 of 396 (6.3%) poultry handlers from live poultry markets had antibodies detected. Only 9 (0.8%; statistically significantly fewer) of the 1129 community members showed signs of an immune response to H7N9 infection while 33 of 45 (73%) laboratory confirmed H7N9 cases had significant levels of antibody.

No poultry handlers (mostly exposed through slaughtering) had H7N9 in nasal swabs collected at the time of blood sampling, probably reflecting that collection had occurred after the infection that elicited antibody had resolved. Less than 4% of poultry handlers or the general community had fever or respiratory symptoms at sampling compared to 100% of the lab-confirmed group.

This partially answers one of my questions from earlier in the year - but leaves the part which asks: if the main H7N9 host is poultry (and not wild birds), why don't we see the majority of ill people coming from the poultry worker population? While aerosol transmission has been described as low among ferrets, H7N9 transmission might be effective enough to explain the other human H7N9 cases not due to slaughtering of poultry.

Now we can say that poultry handlers are getting exposed and 3 in every 50 are getting infected (or mounting an immune response, to be pedantic). Only 3% of this population and 10% of the general community had underlying diseases compared to 64% of the lab confirmed cases. Sex of the groups did not seem to play a role but those aged ≥60-years were over-represented among the lab-confirmed H7N9 cases (53% of them) compared to poultry handlers (1%-a much younger population) or the general community (19%).

As for MERS-CoV, underlying conditions and older age are clearly important risk factors for more severe disease.

The authors also noted that higher antibody levels were found in survivors that in fatalities, perhaps suggesting (a) the fatalities did not have time to mount a suitable response before they succumbed or (b) the antibodies protected against worse outcomes. Poultry workers do not always have serious disease, which probably means lower viral loads and thus reduced likelihood that they are major sources of human-to-human transmission.

In a previous study by Bai et al, using one of same sort of antibody detection techniques (haemagglutination inhibition), no poultry handlers from were found to be positive prior to late 2013. So this new article proves the emergence of H7N9 human infections is a recent event. And this provides Chapter 2 on that earlier post. 

Some questions still remain in my mind:

• Are these 3/50 poultry handlers also getting moderately or severely ill? 
• How often does infection in this group result in asymptomatic or mild disease?
• If disease is mild or asymptomatic in poultry handlers, is it because these workers are exposed to poultry with other influenza viruses comprised of proteins that are or are sufficiently related to H7 and so they already have some protective immunity to moderate their disease after H7N9 infection?