Health officials expect the flu season to continue for several more weeks, and the CDC estimates that overall, the flu vaccine is 36 percent effective at preventing the disease, according to a report released Thursday.
The CDC's preliminary study, which involved approximately 4,600 people, will be updated when the flu season ends.
Here are four things to know.
1. The vaccine's effectiveness against H3N2, this season's dominant strain, was lower than the overall effectiveness, at 25 percent.
2. The report estimated the vaccine was 42 percent effective against influenza B viruses and 67 percent effective against H1N1 viruses.
3. Vaccine effectiveness was 59 percent for children aged six months to eight years and 33 percent for those aged 18 to 49 years. The study found no statistically significant protection in other age groups.
4. The CDC reported last week that approximately 10 percent of hospitalizations in the U.S. are due to flu cases. As of last week, the season had not yet peaked.