WHO swaps out H3N2, B strains in Southern Hemisphere flu vaccine

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At their meeting this week in Melbourne, Australia, World Health Organization (WHO) vaccine advisors today recommended changing two of three components for trivalent vaccines that pharmaceutical companies will produce for the Southern Hemisphere's 2018 flu season.

The group recommended a new influenza A H3N2 strain, and swapped out the Victoria-lineage B strain with a Yamagata-lineage virus that had been recommended last year as the second B strain in quadrivalent vaccine formulations.

The WHO recommends the following for the Southern Hemisphere's trivalent vaccines:

• For H1N1, an A/Michigan/45/2015-like virus
• For H3N2, an A/Singapore/INFIMH-16-0019/2016-like virus
• For B, a Phuket/3073/2013-like virus (belonging to the Yamagata lineage)

For quadrivalent vaccines that contain two influenza B strains, WHO experts recommend adding Brisbane/60/2008-like virus, a Victoria-lineage virus what was a component of trivalent versions of the Southern Hemisphere's current vaccine as well as the Northern Hemisphere's 2017-18 season vaccines.

Seasonal flu vaccines for the Northern Hemisphere's flu season will still contain the earlier H3N2 strain, and its trivalent vaccine's B strain will be the Victoria-lineage virus.

Changes in circulating H3N2 viruses

At the global level, H3N2 and influenza B are the predominant seasonal flu strains, and in the most recent flu developments, Australia is still reporting high flu levels, led by H3N2, with high numbers of deaths in nursing homes and in healthy adults.

Australia's Department of Health said in a statement yesterday that overall, the 2017 vaccines have been a "relatively good" match, but evidence suggests less effectiveness than usual, especially for protecting seniors against H3N2 infection.

In its strain selection report, the WHO said the majority of H3N2 strains that circulated from February to September belonged to the clade 3C.2a and subclade 3C.2.a1, which continue to show considerable diversity in their hemagglutinin and neuraminidase genes.

Most recent viruses are antigenically similar to the cell-propagated version of Hong Kong vaccine virus from the previous season, but were less similar to the egg-propagated virus, according to results from ferret serum tests.

Ferret serum tests showed that circulating strains were better inhibited by antibodies raised against the egg-propagated Singapore H3N2 vaccine virus strain. The group also said the Singapore vaccine strain, part of the 3C.2.a1 subclade, contains the N121K hemagglutinin substitution seen in the majority of recent H3N2 viruses and that the neuraminidases of recent H3N2 viruses are antigenically distinct from last season's H3N2 vaccine component.

Dan Jernigan, MD, MPH, with the US Centers for Disease Control and Prevention's National Center for Immunization and Respiratory Diseases, was asked about the WHO's new H3N2 component recommendation today at an annual news briefing hosted by the National Foundation for Infectious Diseases (NFID) to raise awareness about seasonal flu vaccination. He said there has been a little bit of drift in circulating H3N2 strains but no significant mutation. Jernigan said the new vaccine strain mainly affects egg-based flu vaccine manufacturers.

Regarding influenza B, both lineages co-circulated in varying proportions in different regions, but Yamagata predominated in Oceana, Europe, and the Americas, with Victoria more common in Asia and Africa.