Google Flu Trends is a useful tool in tracking the severity of a given flu season. But take some caution in drawing conclusions from it. Exhibit A: Look at the discrepancy between the following two charts.
The first is Google Flu Trends' plot of national flu activity, compared with the last six years of flu activity. Google compiles this chart by sifting through the search terms that someone might type if they had the flu. In the aggregate, this information is used to create a chart of flu outbreaks that often matches the Centers for Disease Control and Prevention's data on outbreaks.
The bright blue line is the current year — and it looks pretty bad.
Now, look at a chart from CDC's Outpatient Influenza-like Illness Surveillance Newtork. This data is compiled from 3,000 clinics across the country that report to CDC when they see patients with flu-like symptoms. This is one of several graphs included in CDC's weekly flu reports. They also track mortality rates, have a network of labs analyzing viral specimens, survey hospitals for confirmed flu patients, and track the geographic spread of viral strains, among others.
The red line is the current flu season.
If you have a keen eye, you can see the difference. The first chart implies that this is the worst flu season in recent years, while the second shows a severe season but still below the H1N1 (swine flu) pandemic year of 2009-10. Granted, CDC's data runs about a week behind Google's, but even data points from a few weeks back show this discrepancy.