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The World Health Organization, which guessed incorrectly last year about the most troublesome flu strains to fight, has recommended changes for next year’s vaccine supply. The U.S. Food and Drug Administration is expected to adopt the changes Thursday. Seniors and the nursing home population are the hardest hit groups when it comes to flu season.

Flu vaccines consist of three strains, and two of them are expected to change. The Fujian strain that caused so much trouble after being rejected for inclusion last year will replace the strain known as A/Moscow. The other change will be a substitution of B/Shanghai for the B/Hong Kong strain. The A/New Caledonia strain will be a holdover from the last flu season.

The World Health Organization’s monitoring network gathers information on active flu strains around the world. Vaccine candidates for the Northern Hemisphere need to chosen now so they can be properly bred and prepared for use late this year.

Experts said they knew the Fujian strain was beginning to cause problems at this time last year, but there was not enough time or information to include it in the subsequent flu vaccine.