How do vaccines get updated for changing flu strains each year?

Influenza viruses change frequently through mutations and reassortment, so vaccine makers update seasonal flu vaccines regularly. The World Health Organization coordinates global surveillance and recommends strain choices for the Northern and Southern Hemisphere seasons. Manufacturers then grow the selected strains in eggs or cell cultures, or use recombinant methods, before the shots are distributed. Even when the match is imperfect, flu vaccination can still reduce severe illness and hospitalizations.