Flu viruses are mutating masters, always on the move to outwit scientists and their vaccines. This ability to adapt quickly, and the range of flu-virus strains, requires a new vaccine every flu season to match what’s circulating that year.

When we recently asked readers for your questions about the flu, many of you wondered how public-health officials make sure each year’s vaccine is made to stymie that year’s flu strains.

Why your arm hurts so much after a flu shot

Even though the flu is discussed in terms of seasons, it’s always circulating, and it affects populations all over the world, said Dr. John Dunn, medical director for preventive care at Kaiser Permanente Washington.

The decision on which strains of flu to fight falls to the World Health Organization (WHO), with input from the United States Centers for Disease Control and Prevention (CDC). In February, health officials and scientists look at what’s circulating in the southern hemisphere to see what might make its way to the northern hemisphere; the same evaluation is done of the northern hemisphere in September to plan for the southern hemisphere. Flu data from public-health departments, clinical studies and laboratory results are all taken into account.

Once the target strains are chosen, the manufacturing process moves to the private sector, where a handful of companies create the vaccine using one of at least three ways currently approved by the U.S. Food and Drug Administration (FDA).

The most common method uses eggs, as was done when the flu vaccine was first developed about eight decades ago. The viruses are injected into fertilized hen eggs and allowed to incubate for several days before the liquid containing the virus is extracted. The virus is then killed and the antigen is purified and made into the vaccine, which is then shipped to doctors’ offices and pharmacies.

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Two other methods are sometimes used: Cell-based vaccines are grown in animal cells instead of in fertilized hen eggs; recombinant vaccines are made by isolating a particular gene from a flu virus and mixing it with another virus that can grow in insect cells.

When to get the flu shot

When the wrong flu strains are identified or mutations happen (viruses sometimes mutate when grown in eggs), vaccines can be less effective. But even as the effectiveness varies, getting a flu shot each year still reduces your risk of catching the flu by 40% to 60%, according to the CDC. That’s important not only for you as an individual but for your community, as it keeps a mild flu season from becoming a bad one, Dunn said.

“Part of the challenge is that, every single year, we are starting all over, and we got to get everybody in again,” he said.

But an annual flu shot could one day be a thing of the past. Researchers at the University of Washington School of Medicine’s Institute for Protein Design are working on a universal flu vaccine that would immunize people for a lifetime.

Efforts to move the needle on flu-shot rates get stuck