CDC: Flu season has arrived, latest start in 24 years

Fact.  Vaccines aren't risk free. The most common side effects are soreness at the injection site and fever, which are best treated with acetaminophen or ibuprofen. Less common are seizures (defined as "jerking or staring"), and risks vary depending on the vaccine. For example, 1 in 14,000 children suffer a seizure after receiving the DTaP shot; it's 1 in 3,000 with the MMR vaccine. Some kids are at higher risk for side effects than others. In these cases, it may be best to proceed with caution or skip them, according the CDC. More from Health.com: 12 vaccines your child needs istockphoto

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(CBS) Where's flu season? Health officials have been warning about it for months, even telling Americans there's "no excuse" to skip this year's influenza vaccine.

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But according to a new report from the Centers for Disease Control and Prevention, flu season is finally here - the latest it's started since the 1987-1988 influenza season.

Published in the Feb. 24 issue of Morbidity and Mortality Weekly Report, the latest flu report shows influenza activity has slightly increased in February, but is still remains relatively low. The CDC has been monitoring the flu for months, but does not consider flu season officially kicked off until more than 10 percent of respiratory specimens from laboratories around the country test positive for flu. That did not happen until Feb. 4; one week later, the percentage of samples testing positive grew to 15 percent.

The report shows flu has occurred in all 50 states, but notes the spread of flu has been widespread across one state - California. Other outbreaks have been regional in 12 states, local in 17 states, and sporadic in 20 states. Also, flu-related hospitalization rates are relatively low, the report found. For all age groups, about one in every 100,000 people are hospitalized with flu during the current flu season. Last year, that number was 22 out of 100,000 people. Three children have also died from the flu this year, compared to 122 last year.

But just because this flu season is off to a slow start, doesn't mean we're in the clear.

"Although the timing of influenza activity is not predictable, peak activity in the United States most commonly occurs in February; however, substantial activity can occur as late as May," the authors wrote in the report. The authors also said Americans should still get vaccinated, even though flu activity is low.

What explains this season's slow start?

Dr. Lyn Finelli, the head of outbreak surveillance and response at the the CDC, told NPR part of it has to do with the uncharacteristically mild winter we're experiencing.

"We know that influenza virus survives best in cool and dry conditions," Finelli said. A warm winter also means more people are outside, which means fewer people are huddled in close quarters spreading germs to each other.

Another factor may be that this year's main flu strains are the same as last years, so people may have already built natural immunity.

"The combination of natural infections which give you some immunity as well as widespread vaccination, I think we have a very well-protected population at the moment," Dr. William Schaffner, professor of medicine at Vanderbilt University, told NPR.

If you want to track the 2011-2012 flu season, the CDC has a  flu map to show current activity across the country, based on reports from local labs.

For another way to track the flu, check out Google Flu Trends. The search engine tracks flu-related searches, and uses that information to provide a real-time predictor of when people might be getting sick, HealthPopreported.

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