This year's influenza season is beginning to take off. The main strain circulating in the USA is the H1N1 virus, which caused the swine flu pandemic of 2009-2010. It is most dangerous to pregnant women and young children.
High levels of seasonal influenza are being reported in the southeastern USA. Alabama, Louisiana, Mississippi, Missouri, Oklahoma and Texas have reported the highest levels, the Centers for Disease Control and Prevention in Atlanta reported.
This year, there have been 1,156 laboratory-confirmed influenza-associated hospitalizations, and four children have died of flu-related complications, according to the CDC. Adult deaths are not tracked.
Flu season usually begins around now and peaks sometime between January and March, said Joseph Bresee, a CDC flu expert.
"Last year was exceptionally early," he said. In 2012, the fourth week of December was the peak of flu season.
Three flu strains are circulating this year, the most common of which is H1N1.
"This is the first time we've seen it be the predominant strain since the pandemic," Bresee said.
The H1N1 variant was newly mutated in 2009, so few people had any immunity to it. This time around, there's much more population immunity due to four years of vaccinations and illnesses.
Young children are at higher risk because they've had less time to be exposed either through illness or vaccination and because H1N1 has affected younger people hardest in past years.
"It's too early to tell if that will be the case this year, but I wouldn't be surprised if this year, there isn't more disease among young children and young adults," Bresee said.
The flu vaccine this year contains antibodies to H1N1 and is an excellent match, so it should provide good protection, Bresee said.
Pregnant women are a group the CDC is especially concerned about. During the pandemic in 2009-2010, pregnant women were at very high risk of having complications from the flu. It was also dangerous to the babies they carried.
"With the return of the H1N1 virus this year, we want to make sure that pregnant women are knowledgeable about the risks," Bresee said.
Mothers who get vaccinated are also protecting their babies. Since the outbreak in 2009, there have been multiple studies confirming that vaccinating pregnant women protects their babies from flu in the first six months of life, when their immune system is developing and they are especially at risk.
The good news is that this year there is plenty of nasal spray vaccine available so those who don't like needles can be vaccinated with just a squirt of liquid up their nose, Bresee said.
It's not too late to get vaccinated. Though it takes about two weeks for full immunity to occur, the vaccine provides at least some protection within days.
About half of Americans are vaccinated against the flu each year. A new vaccination is necessary yearly because the flu mutates as it circles the globe, and the variants one year are rarely the same as those the next.
There is a persistent myth among some people that getting the flu vaccine gives you the flu, but that's wrong, Bresee said.
"Vaccines don't work that way," he said. The flu virus used in the vaccine has been inactivated, so it cannot cause illness.
Randomized, blinded studies (the "gold standard" of medical research) have shown this to be the case. Some people were given flu shots, and others were given saltwater shots. The two groups had no differences in symptoms beyond increased soreness in the arm and redness at the injection site among people who got the flu shot.
There were no differences in terms of body aches, fever, cough, runny nose or sore throat.
In cases where people get vaccinated, then get sick, it's likely they were either infected with the flu before the vaccine had time to take effect or were infected with another virus that was not the flu.
But the flu vaccine is not perfect. "You can certainly still get vaccinated and still get the flu, but you're much less likely to," Bresee said.