This flu season could be a doozy. The CDC is warning that the flu vaccine isn't as effective this year. The Washington Post's Lenny Bernstein explains why. (Pamela Kirkland/The Washington Post)

This post has been updated.

Anticipating a severe flu season, the government on Thursday recommended immediate vaccination for anyone who hasn't taken that precaution and urged people who come down with the flu to seek anti-viral medication from their doctors.

The flu has already killed five children, and the predominant strain of the virus circulating now was associated with severe outbreaks, including larger than normal numbers of deaths and hospitalizations, during three flu seasons over the past 11 years, the Centers for Disease Control and Prevention announced.

The anti-viral medications oseltamivir (Tamiflu) and zanamivir (Relenza) may be particularly important this year. A CDC study of the flu viruses now circulating showed that the vaccine may have limited effectiveness against more than half of the most common one, H3N2, the CDC said.

Yet fewer than 1 in 6 people infected by the flu virus are treated with anti-viral medications, CDC Director Thomas Frieden said.

Anti-viral drugs "aren't a substitute for vaccine. Vaccine prevents flu," Frieden said. "But anti-virals are an important second line of defense … and this year treatment with anti-viral drugs is especially important."

That is particularly true for children under age 2, adults older than 65, pregnant women and people with other health conditions that leave them more vulnerable to the flu, including asthma, cardiovascular disease and diabetes. Residents of nursing homes and the morbidly obese also should receive the treatment.

Frieden noted that anti-viral drugs work best if they are taken within two days of infection and urged patients and doctors to turn to them quickly. He also emphasized that people with the flu should stay home to avoid spreading it.

Before each flu season, experts try to predict the kinds of virus that will circulate in the United States and drug companies produce vaccines with three or four components to combat them. But the effort is always something of a guessing game and this year, tests  are finding that more than half the H3N2 viruses are antigenically different from the vaccine component, or "drifted" from it, in CDC parlance. The vaccine still should provide some protection against those viruses and will be effective against the others, according to the CDC.

The "drifted" virus was first detected in March, too late to be included in this season's vaccine, Frieden said.

H3N2 viruses predominated during the flu seasons of 2003-2004, 2007-2008 and 2012-2013, when the largest number of deaths were recorded over the past decade. The flu is still at low levels in the United States, but 1,228 confirmed cases were reported to the CDC during the week that ended Nov. 22.

"The flu is unpredictable," Frieden said. This is "likely to be an H3N2 predominant season … and H3N2 predominant seasons tend to have more hospitalizations and more deaths."