There will be an update from CDC later today and WHO’s expert committee established under the new International Health Regulations (IHR) meets via teleconference this morning North American east coast time at 10 am (4 pm Geneva time) to consider whether the swine flu situation merits declaring it ?a public health event of international concern.? If they do, WHO Director General Margaret Chan may respond by raising the pandemic threat alert level from the current phase 3 (new virus: no or limited human to human transmission) to phase 4 (new virus, evidence of increased human to human transmission). It’s conceivable, but not likely they would go to phase 5 (new virus, evidence of significant human to human transmission). Phase 6 is acknowledgement of an existing pandemic (new virus, evidence of efficient and sustained human to human transmission). The level of alert is not just an academic exercise because under the IHR this allows WHO to institute measures like travel advisories which were forbidden before the IHR were revised and went into force last year. For more background on this see our posts here, here, here, here and here.
One of the puzzling things about this outbreak is the stark contrast between the clinical and epidemiological picture in Mexico and the US, with reported Mexican cases winding up on ventilators and dying with severe lower respiratory disease while US cases have been mild with uneventful recoveries. Partial comparisons of the genetic sequences of the Mexican and American cases shows them to be essentially the same (two flu isolates, even from within the same patient, are rarely identical, but most of the changes are not biologically significant). Is this difference real or only apparent? At ProMed, Moderator PC (Peter Cowen, ProMed’s Animal Disease Moderator) makes the following excellent points:
According to the above information are now 8 confirmed cases of the novel influenza A H1N1 virus infection in the USA, all mild, occurring in counties with known increased population flow/transit between the USA & Mexico. According to the official report from WHO, there have been now more than 854 cases of pneumonia reported from the capital (Mexico City) of which 59 have died (case fatality rate [CFR] 6.9 per cent). In San Luis Potosi (central Mexico) there were 24 cases of influenza-like illness (ILI) reported with 3 deaths (CFR 12.5 per cent) and in Mexicali, (near the border with the United States), there have been 4 cases of ILI reported with no deaths.
It should be noted here that the data from Mexico refers to inpatient hospitalized cases, whereas the ILI surveillance sites in the USA are predominantly from sentinel reporting outpatient facilities. This difference in surveillance sites may account for an apparent disparity in severity of the illnesses in cases reported in Mexico vs those presently reported in the USA. As more uniform active surveillance (case finding) is implemented, these disparities may lessen.
The absence of direct connection between the cases in the USA and the cases in Mexico does not rule out the outbreaks being linked, as the population flow between the 2 countries is high and 12 isolates from Mexico are reported to be genetically identical to those isolated in the USA. One suspects the epidemic curve is already multiple generations past the “index case”. The virus has apparently been circulating in Mexico for several weeks, and in the USA for at least 2 weeks according to the above reports. (Comments of Mod PC to ProMed notice of latest MMWR Dispatch)
There are a lot of questions in the Comments threads, some of which I answered as well as I could as we went along, many more that came in during the night. The situation and available information is moving fast, which will answer some questions but inevitably suggest more. I’ll do my best to get to some of them, but I won’t get to all. Many are excellent questions that have no answers . . . at the moment.