By Macy Jenkins

SACRAMENTO, Calif. (CBS13) – It’s flu season and new numbers show the virus is active in almost all of California. Sacramento doctors are seeing patients roll into their offices sooner than expected.

“I can only imagine that it will continue to increase,” said Dr. Lena Rothstein, a pediatric doctor at UC Davis Children’s Hospital.

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Every year is a little bit different. So far this season, only 39% of people in the U.S. got a flu shot.   It’s a number doctors say needs to go up, especially with a particularly nasty strain going around.

The CDC reports flu activity in all 50 states and widespread activity in 12 states including California.  A map released by the California Department of Public Health shows elevated flu activity in all but five counties in the state.

“It’s hard to say,” Dr. Rothstein said, referring to why the numbers are increasing locally.  “Some strains of the flu are just more virulent, or that means that they are a little bit more aggressive, and cause more widespread disease.”

Rothstein told CBS13 she’s seeing flu patients earlier in the season than she did last year.  And she says it’s because we’re seeing cooler than normal temperatures.

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“We actually see a big change in temperature and humidity and that causes the flu virus to spread,” Rothstein said.

And this year’s flu is more aggressive, with one strain in 80% of the cases: H3N2.

“Really rapid onset, high fevers, muscle aches, sometimes congestion,” Rothstein explained.

Last flu season, the vaccine was 32% effective against H3N2 and 39% effective against all strains combined.  But it’s still too early to tell how effective it’ll be this year.  Until then, Rothstein says get your flu shot.

“The best thing you can really do is stay home, protect yourself, and protect your loved ones and everyone around you if you’re sick,” Rothstein said.

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Flu season just ended in Australia and preliminary reports show this year’s vaccine was just 10% effective against H3N2.  But here in the U.S., it’s too soon to tell how well the flu shot is going to work against the aggressive strain.