Flu prevalence last year (left) compared to this year (right), according to CDC flu maps from the last week of December in 2011 and 2012, respectively. Credit: CDC.

This flu season seems especially bad this year now that Boston has declared a public health emergency and a Pennsylvania hospital was forced to construct a tent to handle flu cases. But doctors - backed up by the numbers - say that this season is a shock partly because we had so little flu last year.

Click here to read Dr. Richard Besser's blog on the flu so far this year.

The Centers for Disease Control and Prevention has reported 22,048 flu cases from Sept. 30 through the end of 2012. By the same time last year, only 849 flu cases had been reported nationwide. That's 26 times more flu cases by the last week of this year than by the last week of 2011.

"In an immediate sense; we were a little spoiled last year," said Dr. William Schaffner, chair of preventive medicine at Vanderbilt University Medical Center in Nashville, Tenn. "Last year, we had fewer influenza cases than had ever been recorded before.

Schaffner, a former president of the National Foundation for Infectious Diseases, explained that the dominant virus from 2009 through last year was the H1N1 flu strain, nicknamed swine flu. Although many people came down with it in 2009, most of them had been vaccinated against it by last year, resulting in fewer infected people.

But this year, the flu season started early with a different dominant strain: H3N2.

"This year, we're seeing a lot of H3N2, which you see in the past tends to affect young kids and the elderly more," said CDC spokesman Tom Skinner. "That may be some of the explanation for why we're having more of a severe flu season this year."

Schaffner said children and the elderly are always the most vulnerable to the flu, because they have weaker immune systems than everyone else. Children actually exhale more flu virus than adults when they get sick, and they exhale it longer, making them the "great distributors of influenza virus," he said.

Click here to read our story about why the weather has little to do with the flu.

Seniors are the most at-risk for coming down with flu complications, such as pneumonia, which could turn deadly, Schaffner said. In Boston, the mayor declared a public health emergency after four seniors died after coming down with the virus. It's not clear whether they had pneumonia.

Other at-risk populations include people with underlying heart disease, those who have compromised immune systems or those who are severely obese, Schaffner said.

The best way to protect yourself against the flu is to get vaccinated, Schaffner said, adding that it's not too late.

"This season is stacking up to be a moderate to severe flu season," Skinner said. "How severe, only time will tell."

Despite the 26-fold increase over last year, this flu season is still mild compared to 2009's swine flu epidemic, the numbers show.

When the flu season was first dominated by H1N1, there were almost four times as many flu cases by the last week of December 2009 compared to the last week of December 2012: 80,724 cases in all. Of those, 60,847 were H1N1. There were also 229 pediatric deaths by that time last year compared with 18 so far this year.