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Counting the Cases
A recent CBS News article caught my eye. It looked at major overestimates of H1N1 infection around the country — in combination with the fact that back in July the CDC stopped recommending that states carry out H1N1 tests. I sent the article to the folks at the Oregon Public Health Division, to see if they wanted to comment. Anne Thomas, with the Emerging Infections Program, emailed this response:
Like other states, we were initially very interested to know if the new H1N1 strain was circulating in Oregon, so we spread the word to clinicians that the Oregon State Public Health Laboratory (OSPHL) would provide free testing of specimens collected from patients who had influenza-like illness (ILI, defined as a cough or sore throat in conjunction with fever). Like the rest of the country, within a few weeks we quickly established that the new H1N1 strain was in wide circulation in Oregon.
Once that was established, we felt that it would no longer be useful to continue tracking individual cases in outpatients. The amount of testing overwhelmed the OSPHL, and there were so many cases in outpatients that we didn't have the resources to investigate them all--especially when it became clear in the first few months that the new strain did not cause more severe illness than typical seasonal influenza.
We were quite supportive of CDC's decision to stop investigating individual cases, and our current surveillance goals are to monitor the burden of severe disease (through investigation of hospitalized cases and deaths), monitor the level of outpatients with ILI (through sentinel providers and our partnerships with Kaiser and a network of federally-qualified health centers), and track which strains are circulating in OR by doing the sub-typing for pandemic H1N1 at the OSPHL of hospitalized patients and selected outpatients seen by our sentinel providers.
In a follow-up phone conversation, Dr. Thomas also explained what those "sentinel providers" are doing. Basically, each week 17 or 18 health care providers around the state report the number of patients with influenza-like symptoms as a percentage of total patient visits. Normally, they say that the flu season has begun when that percentage reaches 2%. It's approaching 10% now.
To put this in perspective, a normal flu season peaks in February at 4 or 5%.
(You can download the latest report, and plenty of other flu facts, here.)
Given all of this, I pressed Dr. Thomas on where the over-estimates could have come from. She acknowledged that not all people with flu-like symptoms necessarily have the flu: some might simply have colds, or other upper-respiratory infections. And I asked if perhaps all of the media attention (TOL included!) might be scaring people who normally would stay home with a fever — or soldier on with the sniffles — into visiting a doctor. She said it was possible.