If you still haven’t gotten your flu shot, now might be a good time. Though we’re already in the middle of flu season, prime time for the influenza virus usually lasts until March, and it's not too late to build up your immunity. Plus, this year’s crop of bugs seem to be rather nasty, officials say.

If you’re already experiencing symptoms of the flu -- fever, muscle aches, coughing, runny nose and sore throat -- do everyone in the office or on the subway a favor and stay home if you can.

On Friday, the U.S. Centers for Disease Control warned Americans that the agency is already seeing levels of flu that indicate that 2012-2013 is going to be at least a moderately severe flu season.

“While we can’t say for certain how severe this season will be, we can say that a lot of people are getting sick with influenza, and we are getting reports of severe illness and hospitalizations,” Joe Bresee, chief of the Epidemiology and Prevention Branch in the CDC’s Influenza Division, said in a statement.

Twenty-nine states and New York City are reporting high levels of the flu. In New York state alone, there have been more than 15,000 reported influenza cases -- a 250 percent increase over last season’s 4,400, according to the New York Daily News.

Influenza hospitalization rates are also high, at 8.1 per 100,000 people, according to the CDC. Eighteen children have already died from the flu this season.

Every flu season is a unique beast, with different viruses dominating the landscape from year to year. This year, the predominating viruses in circulation are influenza A H3N2.

“Typically H3N2 seasons have been more severe, with higher numbers of hospitalizations and deaths, but we will have to see how the season plays out,” Bresee said.

Luckily, more than 90 percent of the viruses the CDC has found in circulation are covered by the 2012-2013 flu vaccine, which is the best first line of defense.

However, “it's still possible that some people may become ill despite being vaccinated,” Bresee said. “Health care providers and the public should remember that influenza antiviral medications are a second line of defense against influenza.”

Antiviral drugs like Tamiflu should be administered as soon as possible after a person falls sick. They’re particularly recommended for vulnerable populations at risk for serious complications, like the very young, the very old and people who are already hospitalized, pregnant or with certain preexisting conditions.

The CDC is already starting to analyze how effective this season’s vaccine is against this year’s strains. Scientists are pitting the vaccine against flu viruses in the lab, and the agency expects to publish interim vaccine efficacy results as soon as they’re available.

“These estimates will provide more information about how well this season’s vaccine is working,” Brisee said.