It may not be possible to get a true count of the number of people who had pandemic (H1N1) influenza when the pandemic has run its course—whenever that is—-but still, infectious disease experts want to get as close as they can.

Barriers to accurate measurement include, but are not limited to:

• Test Problems. Researchers just found that the rapid flu test is sensitive to H1N1 only 11% of the time (meaning the test was accurate only 11% of the time when H1N1 was present). The “gold standard” test is accurate only 12% of the time. Therefore, neither test can provide the kind of confirmation of diagnosis that is part of tracking seasonal flu cases.

• Patient Reporting Problems. Many patients with the mild form don’t even call the doctor. Each week, the CDC’s report includes information about visits to doctors’ offices for “influenza-like illness” (ILI). The  patients with mild cases don’t go in and therefore are not counted.

• Global Changes in Reporting Problems: Many countries stopped counting milder cases, causing the global total to be much lower than, in reality, it is.

For all kinds of reasons, it is important to get as accurate a picture of this pandemic as possible.  The history of each pandemic is a source of information for the future. For instance, there was a debate over whether this pandemic would be more like the deadly 1918 one or the one 40 years ago, also associated with swine. Depending upon what an expert looked at and when, there were hints of 1918 to be sure—the deep lung involvement of young patients who then died. That kind of lung problem has been present in many of the pregnant women who have died. On the other hand, the majority of cases were mild, which was more like four decades ago. Besides, we have plenty of tools now that were not available in either of those pandemics. Anti-viral drugs are an example of that.

Pandemics are a lot like hurricanes. You know you have one eventually but not when or why. Patterns from each one can be helpful in analyzing the one facing people at the time.

The economic effect needs to be measured. There can be no doubt that a pandemic is an economic drag. It would be useful to know how much of one so that can be factored in to quarterly growth reports.

The “hot spots” need to be examined closely. Hot spots are places which either experienced the pandemic first or most severely. What, if anything, is different in those places? That, too, is valuable information.

Here is the latest on what experts think about the impact of the pandemic since last April, the official start date.

Bloomberg

“Swine flu may have infected as many as 5.7 million people in an initial wave that swept across the U.S. earlier this year, researchers at the Centers for Disease Control and Prevention and Harvard School of Public Health said. (Emphasis added)

“The number of swine flu patients in the U.S. in spring may have been up to 140 times greater than the reported number of confirmed cases, according to a study published in the CDC journal Emerging Infectious Diseases yesterday. A model used by the researchers to extrapolate total cases suggests 1.8 million to 5.7 million infections occurred from April to July. (Emphasis added)

“More than 414,945 people worldwide have caught the new H1N1 strain since the first influenza pandemic virus in 41 years was discovered in Mexico and the U.S. in April, according to World Health Organization data. The tally is “significantly lower than the actual number of cases that have occurred,” the Geneva-based WHO says, because many countries have stopped counting individual cases, particularly of milder illness.”

Source: Bloomberg News, October 29, 2009

Citation: CDC journal, Emerging Infectious Diseases, Online, Publication to follow, October 28, 2009