A very interesting epidemiological analysis of the first cases of novel A(H1N1) flu in China was posted on ProMED-mail this morning by Dr. Ji-Ming Chen, Head of the Laboratory of Animal Epidemiological Surveillance, China Animal Health and Epidemiology Center, Qingdao. Dr. Chen notes that all 12 of the cases in China were imported via air travel.  He writes, "if the prevalence of the A (H1N1) infection among the international airplane passengers is comparable to that in the departure countries, there should be many more cases in USA and Canada than the official records (more than millions?)."

How can this be?  There is more evidence in Chen's epidemiological analysis.  Of the twelve imported cases, only two were identified as possible cases using airport temperature scanners.  These two individuals were the only patients to complain of discomfort (i.e., flu-like symptoms) on their flights.  It seems quite likely that this particular strain of influenza produces very mild, sub-clinical symptoms in many of its victims.  The implication of this inference is that infection could become very widespread without being noticed by public health officials or the public at-large.