The World Health Organization (WHO) is recommending changing two of the three strains in trivalent influenza vaccines for use next year in the Southern Hemisphere because of signs the circulating strains are less well-matched to the current vaccine.
"Increasing proportions of the recently circulating A(H3N2) and influenza B Yamagata-lineage viruses have undergone antigenic drift since the last vaccine recommendation," the WHO said in a question-and-answer statement accompanying the vaccine recommendation released on Thursday.
The WHO is calling for switching the A/H3N2 and B strains in trivalent vaccines and for quadrivalent vaccines, which contain two B strains. The agency is suggesting no change in the second B component. The A/H1N1 strain in the vaccine should also remain the same.
Because of the preparation time for vaccine viruses and to grow them in eggs or mammalian cell cultures, the WHO recommends the vaccine strains in September for the following year's Southern Hemisphere flu season and in February for the Northern Hemisphere season.
To evaluate the match between flu vaccines and flu viruses, WHO laboratories take serum from vaccinated ferrets and humans and examine how the antibodies in the serum react with the circulating viruses.