At Friday's press briefing on the swine flu outbreak, Canadian Press's Helen Branswell twice asked whether CDC's weekly flu surveillance data showing the uptick in swine flu but also an unexpected prevalence in seasonal influenza was an artifact of increased testing or something new and unusual. CDC's Dan Jernigan was not especially clear, but seemed to acknowledge there was a lot of seasonal flu around:
The CDC said part of the increase is certainly due to the fact that much more influenza testing is going on these days, because of concerns about swine flu. But the agency said it seems that it's not just a case of more testing, but more flu too."We would be expecting to see the season slowing down or almost completely stopped from the kinds of surveillance systems that we normally monitor," said Jernigan, deputy head of the CDC's influenza division.
"But what we're seeing is that there are some areas that actually have reports of the amounts of respiratory disease that are coming into their clinics that are equivalent to peak influenza season, and so that's an indicator to us that there's something going on with the amount of influenza disease out there." (Helen Branswell, Canadian Press)
Here's the record from the virologic surveillance system, showing the new flu cases in weeks 17-18, only half of which are swine flu influenza A (see our post here for a description of this component of the surveillance system):
Source: CDC FluView
Checking the data chart underneath the figure shows two things. The first is that the number of specimens tested is much greater than those at the peak of flu season this year, double in week 17, 50% more in week 18, but the percent of those specimens positive for influenza is lower than at season peak. In other words, many more people are being seen for influenza-like illness and many more of these people are having their specimens sent for testing, something we might expect as a consequence of public anxiety and the intense medical interest in the swine flu outbreak. Moreover the late season uptick is not accompanied by an increase in pneumonia and influenza deaths. At least not yet. There is often a lag between those two measures.
This doesn't mean that the apparent increase in seasonal flu is not a real increase. It means we can't tell if the increase is a surveillance effect or a genuine upsurge in seasonal flu cases accompanying the swine flu cases. The question is of more than academic interest because the timing of seasonal flu outbreaks in one year may influence the appearance of the next year's pattern, although we don't understand the dynamics of this yet.
Until recently we haven't been doing virologic surveillance outside of "flu season" (approximately week 40 of the previous year through week 20 of the next calendar year). Laboratory checking for seasonal subtypes ended somewhere around now. This means we have very little data on flu in the late spring and summer months, a giant hole in the surveillance record.
Once again, under investment in public health infrastructure is coming back to bite us.
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there's a second graph in the data (well, several, but I mean this one:)
http://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/images/image182.g…
clicking on it shows the ILI reports from sentinel doc offices.
I suspect some of the confusion is that the testing may be driven by the news, but the doc visits for ILI less so. So, there's more of something causing ILI, and some of it is seasonal flu, and we know that because there is more testing than usual. But we don't know what last year at this time would have looked like with this much publicity and testing in mid-May.
Dem: The ILI visits may be influenced by the same problem: care seeking behavior, which may differ with public perceptions of what is appropriate for a visit. It is hard to unravel. Both testing and visits may be driven by the news. We just don't know how much.
Wouldn't it be interesting to see a chart like this for each state...with a count of total samples tested, those verified by CDC and then once the state itself was able to test.
Of course, that assumes every doctor in the state is either sending samples in to the state health department...or not.
Together, the testing behavior differences of all the states would perhaps suggest a different interpretation for the chart above.
umm, speaking from the southern hemisphere (and about to travel to the US yet again!) what is known about the relationship between North/South and the flu seasons, and the equatorial regions?
we clearly have an increase in the positives-rate
which is still there if we substract the Mexflu cases.
Maybe people are better selfdiagnosing now,
maybe doctors use better methods to evaluate symtoms
and checking whom to sample.
Could all be a result of increased informing ?!
Just an Australian: They are usually 180 degrees out of phase with each other. But "usually" doesn't count in a pandemic.
I know a late flu is ripping through the schools here (Iowa). Usually that stuff's over by the time you get your team out on the fields in late March in a nice driving sleet. By May the only kids out are turned ankles or somebody going to a wedding. But here is is Memorial Day and I haven't fielded a full team since the second week of April. Kids aren't just out for a day or two, but 5 days or more. And hacking for a week after that.
I remember several years ago we had, what seemed to me, an epidemic of flu in Austin. I sent an email to the CDC at the time and got brushed off since it was summertime. That "hole in the surveillance" looked pretty intentional to me at the time.
I wonder how much of this is doctors simply taking the flu seriously again? I've had two doctors in the past 15 years whom if i simply called and told them i had anything resembling a cold or flu they would just call in an antibiotic perscription and not even ask me to come in the office. I dont know if that was unique or simply a lassie-faire attitude toward these things that perhaps is a result of throughput oriented patiaent care.
We've got a 24-hour bug going through middle Tennessee. My wife had it on Wednesday last week, and my daughter's teacher and 4 of her classmates were out with it Friday. (Funny how 4- and 5-year-olds have no qualms about talking about how often they threw up or where, lol.)