WHO reveals delayed pick for H3N2 flu vaccine strain

After waiting an extra month to decide on the H3N2 strain, World Health Organization (WHO) vaccine advisors today finalized their recommendations on strains to include in the Northern Hemisphere 2019-20 flu vaccines.

Their H3N2 recommendation—an A/Kansas/14/2017-like virus—reflects a change from the current Northern Hemisphere vaccine and is not a component of the Southern Hemisphere 2019 flu vaccines, either.

New subclade rising sharply in US, elsewhere

When making the announcement on the 2009 H1N1 and influenza B components at the end of their meeting in February, the experts noted increased numbers of genetically and antigenically diverse strains, with the number of clade 3C.3a viruses rising in several geographic regions.

In its update today, the WHO said the proportion of viruses falling into that clade has increased substantially since November in several countries in western Europe, Israel, and especially the United States.

H3N2 viruses pose an increasing challenge for experts picking the vaccine strains because of frequent changes in the virus and problems making flu vaccine seed strains.

In its background materials today, the WHO said the delayed decision marks the second time the advisory committee has delayed a recommendation. In 2003, the group also took another month to observe developments with antigenically distinct H3N2 viruses.

Delay could affect vaccine supply…

Delays in announcing recommendations and major changes in recommended strains can increase the risk of delays in vaccine manufacturing process.

The WHO said all stakeholders are working together to minimize the threat of delayed vaccine supply and availability as much as possible. For example, it said one of the steps is intensive work by collaborating labs to ensure that manufacturers have access to a high-yielding vaccine virus for production.

To account for possible delays, the WHO advised countries to prepare for a possible 2- to 4-week delay in vaccine supply and to consider extending time for vaccination programs.

…but probably worth gamble, expert says

Scott Hensley, PhD, associate professor of microbiology at the University of Pennsylvania's Perelman School of Medicine, said on Twitter today that he liked the WHO group's recommendation. He said both 3C.3a. and 3C.2a viruses are currently circulating.

"It is not certain if 3C.3a will continue circulating next year, but if they do, this vaccine will likely have decent VE [vaccine effectiveness]," he wrote.

For the past 2 years, egg-adapted 3C.2a vaccines have had low VE, even against 3C.2a viruses, so it's probably worth the risk of updating to 3C.3a, Hensley said. Another advantage is egg-adaption problems that have plagued 3C.2a viruses don't seem to affect 3C.3a ones, so VE will probably be high if that subclade circulates next season.

"This situation really highlights the importance of basic research of influenza virus evolution," he wrote, adding, "Our 'educated guesses' of what might circulate next year are much better today compared to 10 years ago, but we still have a lot to learn."

See also:

Mar 21 WHO flu vaccine strain recommendation addendum

Mar 21 WHO FAQ

Feb 21 CIDRAP News story "WHO changes H1N1 flu vaccine strain, punts on H3N2"

Scott Hensley Twitter posts

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