Image: AP

By the time everything’s said and done, this winter’s flu season may very well be the worst in several years, and by some metrics it may even surpass the 2009 pandemic. Some experts have blamed the intensity of this year’s flu in part on a particularly poor vaccine, which was mainly based on research in Australia, where the flu season begins earlier. But new preliminary data released by the Centers for Disease Control and Prevention Thursday instead suggests that the U.S. vaccine is about as effective as it typically is against the specific strains out there.

The researchers looked at more than 4,000 patients with an acute respiratory infection who had sought medical care at one of five flu surveillance sites across the country since last November. Of the 1,712 patients who tested positive for a strain of flu, 43 percent had obtained a vaccine beforehand. They found that the overall vaccine effectiveness across all ages and types of flu was 36 percent, after adjusting for all factors.

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“Although more effective vaccines are needed, vaccination prevents a substantial burden of influenza-related illness annually,” the CDC authors wrote in the Morbidity and Mortality Weekly Report.

Breaking it down further, the flu vaccine was about 25 percent effective against the H3N2 strain, which has been the primary headache this season. Research in Australia had earlier estimated the vaccine over there was only 10 percent effective, while similar research in Canada pegged it to be about 17 percent. H3N2 has historically been difficult to vaccinate against, with an average effectiveness of 33 percent over the past decade, thanks to its particularly quick mutation rate.

Flu vaccines produced traditionally also seem to become more incompatible against a circulating strain of H3N2, thank to tiny changes in the viruses grown inside eggs that are used to make the vaccines. Cell-based vaccines, which the U.S. has begun to move toward, seem to suffer less from that problem. And indeed, the CDC found evidence that people given cell-based vaccines were better protected against H3N2.

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The vaccine also protects against the other major strains in circulation this season: It’s 67 percent effective against the influenza A strain, H1N1, and 42 percent effective against influenza B viruses. And it’s particularly good against all viruses among children under the age of eight—around 59 percent. Conversely, it seems virtually ineffective against H3N2 among children and teens ages of 9 to 18. Among older adults, it’s less than 36 percent effective against H3N2.

So far, there have been 63 pediatric flu-related deaths reported this season, though this is almost certainly an underestimation. Of 54 children killed who had been eligible for vaccination (over the age of six months), only 14 had received a flu shot.

In light of all this, and because the flu season still hasn’t reached its high point yet, with weeks left to go, there’s really one piece of advice to give: Get your flu shot.

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[MMWR]