The flu season got a late start this year, but it is finally picking up speed, according to activity on Google Flu Trends, which tracks search terms plugged into Google, not actual flu cases.

Updated February 12, 2009

By Theresa Tamkins
WEDNESDAY, Feb. 18, 2009 ( — The flu season got a late start this year, but it is finally picking up speed, according to activity on Google Flu Trends, which tracks search terms plugged into Google, not actual flu cases.

Overall, 17 U.S. states have high flu activity, 28 have moderate activity, and only five have low activity, according to the search engine. The Centers for Disease Control and Prevention (CDC), which tracks flu cases reported from local health departments, typically picks up on cases a couple of weeks after Google Flu Trends.

The most recent CDC report, from early February, showed widespread activity in 16 states, up from five the previous week. (That same week Google Flu showed high activity in 18 states). In addition, the agency says, four children have died of flu-related complications this flu season, with deaths in Texas, Colorado, New York, and Tennessee.

Using Google Flu to check for activity in your region is “quite a reasonable thing,” says Anthony Fiore, MD, a medical epidemiologist and infectious disease specialist at the CDC. “They are typically ahead of us,” he says. For example, the state of Maine has only regional activity according to flu cases reported to the agency but high activity according Google Flu.

Google Flu Trends was launched to the public in November 2008 as a new way to track flu activity. The site monitors search terms for words such as influenza and flu. In constructing the site, Google engineers checked hundreds of billions of searches over a five-year period to find those that most correlated with actual cases reported to the CDC.

“They’ve been careful with developing the model, back-checking it against what we have, and refining as they need to,” says Dr. Fiore. During last year’s flu season, Google Flu Trends picked up a significant rise in such search terms on January 28, 2008, and two weeks later the CDC saw a rise in cases reported from local health departments.

“So far, it works pretty well,” says Dr. Fiore. However, he says, “It’s important to remember that it's data based on people’s searches for symptoms and information and not actual disease reporting. We’re always concerned that it could be perturbed by some event that causes people to look for things but who don’t necessarily have flu.”

Google reports only on flu cases, and only those in the United States, but the search engine hopes to expand to other countries and illnesses, says Google spokesperson Katy Bacon.

“This is really a first step, and we’re definitely very excited about where this will go,” she says.

This year’s flu season may be getting a later start than last year’s, but it’s no surprise that cases are picking up, says Dr. Fiore. February has been the peak month for flu activity in 11 of the last 20 seasons, he says.

The good news is that it’s still not too late to get a flu shot, he says, and indeed it is a good idea. Experts recommend you get a flu shot as soon as it’s available, typically in November, because the virus can sometimes get its start before Thanksgiving.

However, most people get sick in January and February. While it takes two weeks after you’re vaccinated to get full protection, you probably get at least some protection soon after getting a flu shot, Dr. Fiore says.

“So it's worth still getting vaccinated at this point, especially in a season that’s starting somewhat late,” he says. Even if everyone around you is sick and you’ve stayed healthy, a second wave of flu almost always sweeps through communities, and peak activity can last for six to eight weeks, he says.

“You have to remember, there are three strains of flu virus,” says Dr. Fiore. “Widespread activity right now may be entirely due to one strain, and you could see another sweep through three or four weeks from now.”

Overall, this year’s flu vaccine is a “good match” in terms of protection against the viruses circulating in the community, he says. The vaccine is designed to combat two A-type viruses and one B-type virus. The vaccine is turning out to be good protection against A-types, but not B-type, he says.

“It’s a mixed picture, but I think overall it looks so far like it will be a good match year,” he says. “About this time last year we were finding a lot of differences amongst even the influenza A strains, but it's looking at this point to be better than last year.”

As for Google Flu Trends, Dr. Fiore says, “If its something that makes people more interested in preventing flu because they have a sense that it's coming soon, then it’s all good.”

Update: This story was originally posted on Thursday, Feb. 12 and updated to reflect new information on Wednesday, Feb. 18.