The WHO called for switching the A/H3N2 and B strains in trivalent vaccines. For quadrivalent (four-strain) vaccines, which contain two B strains, the agency recommended no change in the second B component. The A/H1N1 strain in the vaccine also should stay the same, the WHO said.
"Increasing proportions of the recently circulating A(H3N2) and influenza B Yamagata-lineage viruses have undergone antigenic drift since the last vaccine recommendation," the agency said in a question-and-answer statement accompanying the vaccine recommendation.
Because of the time it takes to prepare vaccine viruses and 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.
The WHO recommends the following for trivalent vaccines:
- For H1N1, an A/California/7/2009-like virus
- For H3N2, an A/Switzerland/9715293/2013 virus
- For type B, a B/Phuket/3073/2013-like virus (belonging to the
Quadrivalent vaccines have been developed in recent years because of the difficulty of predicting which B lineage, Victoria or Yamagata, will dominate in any season. Yamagata viruses have remained dominant over Victoria strains over the past 8 months, the WHO said.
Antigenic driftTo assess the match between flu vaccines and circulating flu viruses, WHO laboratories take serum from vaccinated ferrets and humans and examine how well the antibodies in the serum react with circulating viruses.
Most recently isolated H3N2 viruses were antigenically distinguishable from the H3N2 virus in the current Northern Hemisphere vaccine, called A/Texas/50/2012, and more closely matched A/Switzerland/9715293/2013, the WHO said.
The strains in the current Northern Hemisphere vaccine are the same as in last year's version and in the 2014 Southern Hemisphere vaccine.
Low drug resistanceIn a note on drug resistance, the WHO report said the "vast majority" of H3N2 and B isolates analyzed in recent months were susceptible to available antivirals, including oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (sold in Japan as Rapiacta), and laninamivir (sold in Japan as Inavir).
It said most H1N1 viruses also were susceptible to the drugs, but a small proportion of tested H1N1 isolates showed resistance. In Japan, 2.7% of isolates showed resistance to oseltamivir and peramivir.