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.
The dark line is also complicated. It is the percent of all specimens that are positive. You can think of it as the ratio between the bar chart you see and one you don’t see: the total number of specimens submitted every week. It is subject to the same factors (the chance of a case getting a specimen taken and the chance the specimen is actually flu) but here the two factors are separated. You can imagine the number of specimens going way up but the number of positives staying the same (e.g., if the criteria for submitting a specimen were loosened or lab capacity suddenly increased to allow it; or the reverse). In that case, submitted specimens would increase without flu increasing and the bars would be the same height but the dark line would go down.
Both of these measures (the scale for the bars is on the left vertical axis, for the dark line, the right vertical axis) are affected by the amount of flu circulating in the community but they show slightly different aspects of it. Neither is what you want for an epidemic curve: the risk of contracting flu in a time interval, although both are affected by this risk (and other things), in a complicated way.
Still, as a surrogate measure they are useful and we see a general pattern here. The end of normal flu season has given way to a second flu season with novel H1N1. Trying to make fine quantitative comparisons between the two seasons is risky for the reasons given and neither hump is an epidemic curve. But that we are seeing a recrudescence of influenza is clear.
Welcome to the pandemic.