What looked like probable human H5N1 in Vietnam with four cases in one family (a mother and three children), may not be, although the circumstances are sufficiently suspicious we prefer to suspend judgment a bit longer before concluding that this was a false alarm. False negatives and false positives occur. In some senses, though it doesn’t matter much. The virus continues to be found in poultry in more and more places in Vietnam, with 73% of poultry samples in the central province of Qung Nam reported positive. These developments are probably a combination of endemic virus, infected poultry smuggled in from China to meet the demands of the upcoming Tet holiday and possibly some role for migrating wild birds. Human and poultry cases follow a typical seasonal pattern of influenza, so we will likely see human and poultry cases as the season advances.
Why flu is seasonal, is a bit of a mystery. Various explanation have been given but none have been shown to explain the pattern to everyone’s satisfaction. We are also entering flu season for “ordinary” human influenza, and we took a look at the latest CDC report and found a bit of a surprise. The most recent report is from week 51 (the week before this one) and some of it is quite predictable. Using data from specimens tested by WHO and national reference laboratories in the US, we find four states with widespread influenza activity, 12 more with regional activity, 5 states with local activity, one state reporting no activity and 3 states who did not report. Here’s the map. You can see the widespread activity is in the southeast states:
Source: CDC weekly flu report
The big surprise (to me, anyway) was in the antigenic characterization of the circulating virus this season. Since 1968 H3N2 has been the dominant subtype but in 1977 it was joined by H1N1, which had been the principal subtype between 1918 and 1957. The usual circulating influenza A was H3, however. This year, almost all the typed influenza A virus has been H1. Only one isolate (in week 42) was H3. We should say that most of the viruses (77) were untyped, so it is almost certainly true that some of these were H3s, but the lopsided proportion (24 H1 versus 1 H3) in typed viruses is quite striking and unlike previous three years where H3 predominated, overwhelmingly. The strains in both cases matched the seed strains in the vaccine, so it doesn’t appear that the H1 predominance is vaccine strain mismatch.
With all the attention on H5N1, it is wise to remember that the greatest influenza danger at the moment is from the old stand-bys, influenza A subtypes H1N1 and H3N2 and influenza type B, the viruses responsible for “usual” seasonal influenza. What this seemingly unusual pattern of circulating H1N1 early in the season might mean (if anything) I don’t know, but it is curious. Influenza ramps up noticeably in January of most years, so there is still time to get a flu shot, which is well matched for these viruses.
While we struggle to understand what might spark a pandemic, we still have to contend with endemic influenza which itself is capable of explosive outbreaks that go beyond the seasonal baseline. In 1951 a seasonal H1N1 outbreak in Liverpool produced mortality there that exceeded that of the 1918 variant, so this virus is anything but benign, and we still don’t understand it.
While your eyes are on Vietnam and Indonesia, don’t forget what’s happening where you live.