Tuesday, December 8, 2009

Flu Decline Continues

U.S. flu reports peaked shortly after the flu hype at the end of September and beginning of October and have fallen week by week ever since. Actual U.S. flu cases appear to have peaked in the second half of August. The most encouraging news, though, is the reason epidemiologists believe flu is now declining.

Flu cases are declining in the United States, they believe, simply because so many people have already had the new H1N1 flu. About 30 to 40 percent of people have had the flu already, with most of them not reporting any symptoms at all. A large number, approaching 20 percent, have been vaccinated. Another group, roughly half as a rule of thumb, are naturally immune to the new virus. There is considerable overlap among these groups, but any way you look at it, the pool of people available to spread the virus is much smaller than it was, with statistically more limited chances for the virus to spread from person to person. In simple terms, if you are worried about the H1N1 flu, you have probably had it already, and just didn’t notice it.

The number of flu cases reported in the next three weeks will surely increase, as the Department of Health and Human Services launched a new flu awareness campaign yesterday. The resulting hype will lead to more doctor visits and more testing, and therefore more flu reports. Health officials will point to this increase to justify their flu hype. But it will not mean that the actual number of flu cases is increasing unless the increase is confirmed by other measures.

This 2009 H1N1 flu spread so quickly in part because it was so mild. Only a small fraction of people with the infection showed obvious symptoms, and people who don’t know they have an infection are more likely to spread it. Epidemiologists now say it is certain to be the mildest pandemic ever recorded.

Quite obviously, though, milder pandemics occur all the time and go undetected. The H1N1 flu pandemic could easily have been missed if it had not showed up in such a concentrated way in Mexico City, New York City neighborhoods, and U.S. universities. The hot spots aside, there was nothing in health statistics to make researchers look for the virus. This is one reason epidemiologists believe the H1N1 virus could have been around for 3 to 5 years before it reached Mexico City, perhaps infecting only a few people around factory farms.

The ultimate toll of the H1N1 flu pandemic is now estimated to be a small fraction of one regular season of flu. As scientists get quicker at detecting new infections and viruses there will be more pandemics reported, and we will come to understand that most pandemics are nothing to worry about.