WASHINGTON — In early March, Dr. Robert Daum and other infectious-disease specialists from around the country will gather in a Silver Spring, Md., hotel to choose the influenza strains that vaccine makers should target for next year’s flu season.
It’s an annual medical guessing game of sorts, one backed by data but also plagued with uncertainty. And when the guesses don’t exactly match the reality, as happened this past year, it can mean a dismal and deadly flu season.
‘‘We’ll do the best we can,’’ said Daum, a Chicago doctor who heads the Food and Drug Administration (FDA) advisory committee that makes the recommendations. But ‘‘the virus is smarter than we are at this point. I don’t know of any disease that plagues us more. It’s very, very frustrating and a very inexact science. . . . We do it with varying luck, and I think the luck is mostly the virus’s whim.’’
As it does each year, the group will pore over surveillance information from around the globe, hear presentations from government researchers, and weigh recommendations from the World Health Organization.
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The specialists will cast their votes for the four specific flu strains — two each from the ‘‘A’’ and ‘‘B’’ types of the virus — that manufacturers should focus on in making the coming season’s vaccine. Then, they will wait and hope.
Daum said he suspects he will leave feeling the way he so often has in the past — head hanging, discouraged, wishing there was a more reliable way to protect people from the yearly scourge of the flu.
Despite constant tracking and surveillance of the virus in labs across the world and the work of hundreds of specialists at universities, the WHO and agencies such as the Centers for Disease Control and Prevention (CDC), picking the correct flu strains still involves a measure of good fortune. Every few years, specialists miss the mark.
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‘‘It’s inherently hard,’’ said Anthony Fauci, director of National Institute of Allergy and Infectious Diseases at the National Institutes of Health. ‘‘The one thing the flu is, is unpredictable.’’
Global health specialists had barely finished making their predictions last year when one of the strains they chose, a virulent type of H3N2 influenza, began to morph. ‘‘No sooner than the wheels started turning than we started to see a glimpse of a different H3N2 emerging,’’ Fauci said. This ‘‘drift,’’ as specialists call it, rendered this season’s vaccine far less effective than initially expected.
The CDC announced on Thursday that this year’s flu vaccine has just a 23 percent effectiveness rate, one of the lowest in a decade.
In a typical year, when the strains chosen match well with the vaccine produced, people who get flu shots are on average about 60 percent less likely to become so sick that they require a visit to a doctor, according to the CDC.
Such viral drift has been a persistent problem over the years, although less devastating than the ‘‘antigenic shift’’ that occasionally occurs, creating an entirely new strain that leaves much of the population largely defenseless. That’s what led to the 2009 flu pandemic.
Still, this flu season has officially crossed into epidemic territory and could prove particularly severe; the CDC said recently that 43 states are experiencing ‘‘high or widespread’’ flu activity, with a growing number of hospitalizations and deaths. And the worst could lie ahead.
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A big part of the challenge each year is timing.
Vaccine manufacturers face a constant race to create and churn out enough doses to distribute throughout the country ahead of the annual flu season.
‘‘It can’t just be done overnight,’’ said Dr. David Greenberg, vice president and chief medical officer at Sanofi Pasteur, which produces about 65 million doses of flu vaccine each year. ‘‘It’s a very busy process.’’
Every February, the WHO identifies which strains in the Northern Hemisphere are most likely to wreak havoc the following flu season; the FDA’s recommendations, which historically align with the WHO’s, come soon afterward.
After that, drugmakers develop formulations for each strain, and regulators ensure vaccines from numerous manufacturers are safe and similarly potent. ‘‘Standardization is critical,’’ said Jerry Weir, director of the FDA’s Division of Viral Products.
Manufacturers also must produce and package millions of doses and distribute them to physicians’ offices and pharmacies in time for vaccinations to begin in the fall, ahead of the flu season.
The perpetually tight timetable forces specialists to make choices about the next flu season even before the current one has faded. It’s an educated guess, for sure, that includes data about which strains have dominated in recent years and which are picking up in the Southern Hemisphere and likely to migrate north.
But until better predictive models, universal flu vaccines or significantly faster manufacturing come along, the guesswork remains. So does the frustration when the call is wrong.
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