The World Health Organization (WHO) yesterday (Sept 25, 2014)
recommended changing two of the three strains in trivalent influenza
vaccines for use next year in the Southern Hemisphere, because of
signs that changes in circulating strains have made them less well
matched to the current vaccine.
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
Yamagata lineage)
For quadrivalent vaccines, the agency recommended using the same
strain as in the current Northern Hemisphere vaccine for the second B
strain: B/Brisbane/60/2008, which belongs to the Victoria lineage.
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 drift
To 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.
"Current vaccines containing A/Texas/50/2012 antigens induced
antibodies in humans that reacted less well to A(H3N2) clade 3C.3a
viruses," the agency 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 resistance
In 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.
