Influenza season, part 2

CDC has another snapshot of what the flu surveillance system is seeing up through week 23 (ending June 13). It shows flu still circulating in many communities at a time when most seasonal flu is normally at a very low level. Indeed of the 2765 specimens tested in CDC’s network of 150 laboratories, virtually all of the roughly 40% were influenza A (seasonal influenza B has all but disappeared; the others were not influenza). Not all the flu A viruses were or could be subtyped, but of those that were or could be, 98% were novel H1N1. IN other words, there’s lots of flu around, but essentially none are the seasonal strains of last winter. They are almost all pandemic H1N1. Here is a bar chart of positive specimens by week:

Strictly speaking this is not what we would call an epidemic curve (the time course of an epidemic), because neither of the two things depicted (the bars or the heavy black line) are the numbers of new cases of flu in a time interval. Let’s take the bars, first. The bars are positive specimens submitted to laboratories. The chance a true case in the community is counted here depends on several factors. One is whether the specimen will be submitted at all, i.e., whether a practitioner decides it is worth while to swab a patient and submit it to the laboratory. Once submitted, a lab may make its own decisions depending on workload, priorities and other factors. When the flu index of suspicion is high, as it is when there is public attention, practitioners are more likely to submit specimens. Whether a case of illness will be counted in a bar also depends on the chance that someone with symptoms or signs that are otherwise fairly non-specific will actually have influenza. The chance of all of these factors being true increases as flu circulates in the community, but may do so in complicated ways that make it difficult to compare one week with another. There may also be geographic variations that offset or add to each other for the final national tally.

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