The flu has been spreading to children and elders across California and the rest of the United States recently, due to a mismatch of the virus strain in the flu vaccine.

The United States expects an annual flu epidemic, but this year’s outbreak is much more severe. Young children, adults older than 65, and people with weak immune systems are the main victims of this year’s most common flu virus. Spread mostly through coughing and sneezing, flu activity has traveled to 43 states  within the last few months. There have been 21 reported pediatric deaths this season; six of the reported deaths were during the last week of December.

According to the Centers for Disease Control and Prevention (CDC), surveillance shows an increase in flu circulation and hospitalization rates. Doctors blame this increase in flu activity on the mismatching of strains in the flu vaccine issued to the U.S. citizens. When scientists created the flu vaccine, it was designed to combat a certain H3N2 virus.

After taking samples from various flu patients, doctors discovered that this original H3N2 virus mutated. The virus in the patients was different than that in the vaccine. As a result, the flu is spreading faster.

“[The viruses] are different enough that we’re concerned that protection from vaccinations against these drifted H3N2 viruses may be lower than we usually see,” CDC chief Tom Frieden said in a press call with the Washington Post.

Creating a new vaccine is unlikely, since the production would take around four months. Although the current vaccine is not as affective, many doctors still recommend it.

“The flu is bad, and you want to do anything you can to prevent getting it and to prevent giving it to other people,” said Dr. Lisa Thebner, a pediatrician in New York City in an interview with the Cable News Network. “The vaccine isn’t perfect, but it’s the best protection we have for prevention.”