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.

The 2016 flu shot was effective 48 percent of the time, according to the Centers for Disease Control and Prevention (CDC), which is on par with other recent years.

However, "flu is fickle," says William Schaffner, an infectious disease expert at Vanderbilt University and medical director at the National Foundation for Infectious Diseases. And 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.

Trying to predict whether the flu vaccine will be more or less effective in a given season is tricky business. One strategy scientists use is to see what happens in the Southern Hemisphere's winter season, which just finished. Dr. Kanta Subbarao, director of the World Health Organization’s influenza collaborating center in Australia, told STAT News that the flu season there "started earlier than usual, lasted a bit longer, and all segments of the population were affected, including people who were vaccinated."

The number of flu cases are also up in tropical locations, including Hong Kong, STAT News reports. But does this signal a dangerous flu season for us this winter? “That’s the million-dollar question,’’ Sabbarao said.

Strains and mutations

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.

In 2014, the dominant strain of flu mutated after the flu vaccine had already been formulated. It was a particular strain of H3N2 virus called the "Switzerland variant." That's why the vaccine was only about 13 percent effective against that particular strain, said former CDC director Dr. Tom Frieden. It was not a close match to the H3N2 strain found in the vaccine, but the vaccine has since been updated to better match that strain, said Frieden.

However, the H3N2 strain is the virus that made most people sick in the Southern Hemisphere in recent months, and it was the dominant strain in the 2016-2017 season, so it may be headed our way again. The good news is that if you got sick with it last year, you're less likely to get sick this year, Jacqueline Katz, deputy director of the influenza division at the CDC, told STAT.

Flu FAQs

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

  • 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.

How the flu vaccine is made

Most flu vaccines are made from viruses grown in fertilized chicken eggs. This is why you may be asked if you have an egg allergy before getting a flu shot. But researchers have been raising concerns about this production method for a while.

“We’ve got to get out of chicken eggs,” Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, told Scientific American. However, “that’s not easy to do, and that’s expensive — very expensive.”

A new 2017 study found this method of producing vaccines may be decreasing their effectiveness, because the flu strains mutate and adapt to grow inside the eggs. Sometimes those mutations are significant enough to change the effectiveness of the vaccine. Researchers found that vaccines not grown in eggs were more effective against flu strain mutations.

Editor's note: This story has been updated with new information since it was originally published in September 2015.

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