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)

So it turns out this season's flu vaccine was kind of a dud. Getting it reduced a person's chance of having to visit the doctor because of the flu by only 23 percent -- and possibly even less for many adults -- according to data released Thursday by the Centers for Disease Control and Prevention.

"Commonly, it's been closer to 60 percent," said Brendan Flannery, an epidemiologist at the CDC's influenza division, who said this year's figure is nearly the lowest the agency has seen during the decade or so since it began tracking annual vaccine effectiveness.

By now, the reason behind this year's lackluster vaccine seems fairly clear. Not long after global health experts met in February to choose strains to include in the coming season's flu vaccine, the specific type of H3N2 virus they chose began to morph, or "drift." By the time the flu season rolled around in the Northern Hemisphere, more than two thirds of the H3N2 viruses making people sick no longer matched the formulation in the flu vaccine.

The result has been what the CDC calls a "moderately severe" flu season thus far. Infections have been widespread in nearly every part of the country, with thousands of hospitalizations and at least 26 pediatric deaths.

http://www.washingtonpost.com/posttv/national/health-science/cdc-it-may-shape-up-to-be-a-severe-flu-season/2015/01/12/0723654c-46ae-4a72-bec5-738150a24273_video.html

To determine the effectiveness of this season's vaccine, researchers studied data from 2,321 children and adults who visited doctors in five different states -- Michigan, Pennsylvania, Texas, Washington and Wisconsin. In years when the flu vaccine matches well with the dominant virus circulating, most people with confirmed flu cases tend to be those who didn't get vaccinated.

"That’s not the case this year," Flannery said. "Even in people who got vaccinated [there was the flu]."

Even as they acknowledge the shortcomings of this year's vaccine, public health officials have insisted that those who get vaccinated are still better off. After all, the vaccine does protect against multiple strains of flu, including types that could still surface late in the season, and can make symptoms milder even in people who get sick. In its report Thursday, the CDC noted that even a vaccine that's only 10 percent effective could prevent an estimated 13,000 hospitalizations in older people over the course of a flu season.

Given the severity of this year's flu, CDC officials also have repeatedly urged people who get sick to seek influenza antiviral medications, which can lessen symptoms, shorten the duration of the infection and help prevent more serious complications such as pneumonia. "Antiviral medicines save lives, but unfortunately they're underutilized," CDC director Tom Frieden told reporters in recent days.

It won't be long before experts in the United States and overseas will meet again to decide what flu strains manufacturers should target in their vaccines for the 2015-2016 flu season. They likely will include this year's "drifted" H3N2 virus into next year's formula. If it remains a dominant strain next year, the flu vaccine likely will be more effective. But it's an educated guess, at best.

"The only thing predictable about the flu," Flannery said, "is its unpredictability."