As temperatures drop and germ-sharing season kicks into top gear, the talk turns to flu season and the effectiveness of the flu shot. For just a few dollars and a quick stop at the doctor's office or the grocery store, you can ward off days of bed-bound utter misery — sometimes.

Last year's flu shot was a bit of a bust.

How well the flu vaccine works can vary significantly from flu season to flu season. Each season, researchers create the flu vaccine designed to protect against the main flu viruses research indicates will be most common during the upcoming flu season.

"Flu is fickle," William Schaffner, an infectious disease expert at Vanderbilt University and medical director at the National Foundation for Infectious Diseases, said in a recent news conference about the upcoming flu season. "During most flu seasons, flu vaccines offer good protection against circulating viruses. Last season was unusual. Not only did we have one strain of influenza that caused almost all of the reported flu cases, but it was different; it had mutated. It was different than the strain that was represented in the vaccine."

Last year, the dominant strain of flu mutated after the flu vaccine had already been formulated. That's why the vaccine was only about 13 percent effective against that particular strain, said Centers for Disease Control and Prevention director Dr. Tom Frieden. The flu vaccine is usually about 50 percent to 60 percent effective.

This year's vaccine should be much more successful, according to health officials.

There are three kinds of flu viruses that commonly circulate among people today, according to the CDC: influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses. Last year's dominating mutating strain was a particular strain of H3N2 virus called the "Switzerland variant." It was not a close match to the H3N2 strain found in the vaccine and that's why it was so ineffective. This year's vaccine has been updated to better match that strain, said Frieden.

The H3 type of flu also tends to make seniors sicker than other strains of flu, according to Frieden. And last year saw the highest flu hospitalization rates ever documented for people age 65 and older.

The vaccine was, however, successful in combating other non-H3 strains of the flu. It protected against influenza B viruses (which circulated later in the season) by 55 percent, according to the CDC.

person getting the flu shotThe flu vaccine is typically 50 percent to 60 percent effective, according to the CDC. (Photo: CNK02/Shutterstock)

How the flu vaccine is made

Before the flu season starts in the U.S., the CDC is already tracking flu viruses in the Southern Hemisphere, where the flu season runs from April through September, according to Live Science. Researchers use that data to predict which strains will be most dominant.

Data so far this year indicates that the vaccine is on the right track.

"A new strain shows up, it continues to be a dominant strain for a few years. We have watched in our summer what has happened in the Southern Hemisphere in their winter," said Schaffner, according to ABC News. "Their dominant strain has been the same, the H3N2 strain."

So far, data indicates that this year's vaccine should be a good match against this year's circulating viruses, Frieden said. More than 40 million doses of flu vaccine have already been distributed throughout the U.S. And Frieden said the time it takes to make the flu vaccine has been shortened so that it can be made later in the year when researchers have more up-to-date knowledge about flu strains.

But "influenza is always changing," Frieden said, pointing out that the CDC will keep tracking the flu and looking for any changes in the circulating strains.

Flu FAQs

  • The CDC recommends the flu shot for anyone 6 months or older, with few exceptions. The nasal spray vaccine is approved for people ages 2 through 49.
  • Flu season can start as early as October and can stretch as late as May. Cases usually peak between December and February.
  • The flu virus doesn't work the same for everyone. It's most effective for healthy adults and older children. Some older adults and people with chronic illnesses may have more trouble fighting the flu, even with a vaccine, than healthy adults and older children.
  • Even though the flu vaccine may be less effective in older adults, some protection is better than no protection, according to the CDC. People 65 and older are at higher risk of becoming seriously ill or dying from the flu.

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.