Why is the flu vaccine sometimes more effective than other times? Elizabeth Tracey reports

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You may recall hearing that this year’s flu vaccine was 40-60% effective against the flu. Why isn’t it more effective? Anna Durbin, a vaccine expert at Johns Hopkins, explains that’s a function of the process that’s used to make the flu vaccine each year.

Durbin: In the southern hemisphere they have their flu season during our summer. There's a big meeting at the World Health Organization and they look to see what flu strains were circulating, then they make their recommendation about what flu strains are going to be incorporated in the vaccine. Then as we get more data the FDA has a meeting and then they come up with a recommendation of what flu strains should be included. That information is given to the manufacturers.     :30

Because the lead time to make flu vaccines is months long, what turns up as the circulating strains in the northern hemisphere may be different than what was seen previously in the southern hemisphere. And flu viruses continue to mutate all the time, adding complexity to predicting vaccine composition. At Johns Hopkins, I’m Elizabeth Tracey.