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Virus verification in a hen's egg.
In November 2012, as an early and severe flu season bore down on North America, the news from the US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, was reassuring. "So far, this season, most (90%) of the influenza viruses … are well-matched to the 2012–2013 influenza vaccine; this should mean that the vaccine will offer good protection," the agency stated.
Yet vaccine effectiveness against H3N2, the main flu strain circulating that season, proved to be only 46% in adults aged 18–49, 50% in those aged 50–64, and a dismal 9% in people aged over 65, a vulnerable group. Flu hospitalizations and deaths were the highest in almost a decade. What went wrong?
Research published on 25 March in PLoS ONE1 by Danuta Skowronski from the BC Centre for Disease Control in Vancouver, British Columbia, and colleagues at other Canadian public-health research centres, shows that the H3N2 vaccine strain selected by the World Health Organization (WHO) was indeed well matched to the wild viruses circulating at the time. But the strain sent to vaccine makers — which is first adapted to grow better in the hens' eggs used to produce the vaccine — was mismatched and poorly effective, they find.
The likely cause, the scientists found, is three mutations in the egg-adapted strain that were not present in the WHO strain, resulting in changes at key sites of the haemagglutinin surface protein known to affect antibody responses.