(CNN)- Last year's seasonal flu vaccine effectiveness was just 42%, the US Centers for Disease Control and Prevention estimated. Even if vaccinated, people had inadequate protection against the flu.
This limited effectiveness was due to a mutation that occurred in the influenza A (H3N2) vaccine strain, according to a study published Monday in the journal Proceedings of the National Academy of Sciences. This vaccine mutation resulted from an egg-based manufacturing process commonly used today.
This year's flu vaccine may also be imperfect, said Scott Hensley, author of the new study and an associate professor at the University of Pennsylvania. Add to that, he said, "this could end up being a pretty bad flu season." Still, he said, "our best protection" against severe illness is getting vaccinated with the flu shot we have today.
Each year before flu season begins, a vaccine is made based on whichever virus strains are expected to circulate. The selected seed strains are distributed to vaccine manufacturers, which produce their formulations and make them available to health care professionals before the season begins.
During the 2015-16 season, vaccine effectiveness was 47%, but for the 2014-15 season, effectiveness was just 19%, according to the CDC. While the overall effectiveness of last season's vaccine was 42%, it was only 34% effective against the H3N2 viruses that dominated the season.
Vaccine effectiveness varies based on how well it matches the circulating virus strains. Sometimes, a vaccine corresponds to the predominant virus, yet its effectiveness is still not what scientists would expect. Trying to understand which element of the vaccine failed is difficult.
It's too early to speculate which viruses will become dominant in the United States over the course of the coming flu season, Hensley said, "but it's starting to look like it will be H3 viruses." H3 viruses are influenza A viruses.
"There are the A group of viruses and the B groups," said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University. "The A groups are the ones that usually are responsible for large epidemics ... whereas the B flu strains usually smolder along. They always cause illness -- it can be just as severe as the A strains -- but they don't produce large outbreaks."
Though last year's vaccine was mostly ineffective in thwarting the flu, it still prevented nearly 30% of hospitalizations that might have resulted, according to CDC calculations. For older adults, that rate was even higher, at 37%. Plus, the vaccine reduced outpatient visits by 42% last season.
The CDC advises everyone 6 months and older to get a flu shot, as only injectable flu vaccines are recommended. More than 130 million doses of flu vaccine have been distributed so far this year and flu activity is still low across the nation.
It's still early days, but experts believe we may be facing a tough season, and not only because of vaccine concerns.
The reason? Australia had a tough flu season this year, with a total of 215,280 laboratory-confirmed cases and 504 flu-associated deaths reported to its National Notifiable Diseases Surveillance System as of October 13, government data show. According to a surveillance system report, adults over the age of 80 and children between 5 and 9 years old have been most affected.
Health experts urge everyone to get their flu shot. "The other components of the vaccine, like H1N1 and influenza B, will likely provide excellent protection," Hensley said. "The vaccine will also likely prevent severe disease and death caused by H3N2 viruses, even though this component of the vaccine is mismatched."