H1N1, represented here, was the dominant flu strain in 2009 and is again this year.
In 2009, the terms "H1N1" and "swine flu" struck fear as the pandemic caused about 60.8 million illnesses in the United States, resulting in approximately 12,270 deaths.
Once again, H1N1 is the dominant flu strain, accounting for about 95 percent of the flu this season. And following the pattern set in '09, H1N1 is hitting a younger crowd than most flu strains: More than 60 percent of hospitalizations are people between the ages of 18 and 64 according to the Centers for Disease Control and Prevention -- and the flu has caused 20 pediatric deaths so far.
Why the resurgence, especially considering that this is the fifth year that the H1N1 strain has been included in the flu vaccine? For one thing, 16 million people have been born since 2009.
"We don't do a good job of getting them all vaccinated," said Dr. Jonathan Temte, chair of the U.S. Advisory Committee on Immunization Practices and a professor at the University of Wisconsin School of Medicine and Public Health.
And while millions were sick in 2009, millions more were not -- meaning they never developed antibodies for H1N1. Combine that with the fact that only 40 percent of Americans get flu vaccines, and it's not enough for herd immunity. Still, it's not likely to reach 2009 levels, experts said.
"The first time a pandemic virus arrives, no one has any resistance, so it's often huge and it seems everyone gets it," said Dr. Jim Nordin, a clinical researcher studying flu vaccine who practices at a Health Partners clinic in St. Paul, Minn. "Now that it's been around it's acting more like standard flu except for the age range with younger people getting sicker."
Also, the current outbreak hasn't followed the same epidemic curve of 2009, Temte said.
"I would consider that reassuring in comparative terms," he said. "That said, we still see this virus' reach, and its sometimes tragic consequences."
The silver lining is that flu season may peak soon. A cold late November is often a "fair to moderate predictor," Temte said, when compared to the 25-year average influenza curve. "If I were to predict, I'd say it would probably peak out in late January this year."
While there are some patients who get fairly mild symptoms with H1N1, Nordin said that is hotly debated. "Some say once you get it, you get it," he said.
If you haven't gotten it yet, here's what you should know:
*Pregnant women are at high risk: H1N1 not only seems to produce a more severe illness in pregnant women, but it appears to cause a four- to five-fold increase in premature birth, stillbirth, or perinatal death, Temte said. Pregnant women can get vaccinated (and the vaccine will likely provide some protection to the baby as well).
*You can still get vaccinated: The vaccine lowers your chance of getting the flu by 60-65 percent. And if you do get it, your symptoms will likely be mild.
*Antivirals can control symptoms: If you can get a prescription early (ideally, within hours of onset), medication can lessen the severity of symptoms.
*A patient with a "classic presentation" of flu will be able to tell you the hour they started getting sick, Nordin said. "It hits like a ton of bricks."