And if that were not sufficiently unwelcome news, the authors further note that preliminary data put the H3N2 vaccine efficacy at about 10%. This is the same vaccine currently being used in the United States.
One factor in this dismal efficacy is thought to be a mismatch between strain-specific vaccines recommended by the WHO and the circulating influenza strains. However, "[e]ven in years when influenza vaccines are well matched to circulating viruses, estimates of vaccine effectiveness range from 40 to 60%, which is lower than that for most licensed non-influenza vaccines," Dr Paules and colleagues write.
A second factor for low efficacy is that most influenza-vaccine viruses are propagated in eggs, and the vaccine virus changes during egg-based production in ways that facilitate replication in eggs but reduce vaccine effectiveness against circulating virus.
A key change appears to be a mutation in the hemagglutinin (HA) protein that mediates receptor binding by neutralizing antibodies, according to a recent study from researchers at the Perelman School of Medicine, University of Pennsylvania, Philadelphia. The researchers identified an HA glucosylation site that was lost during egg adaptation but remained in circulating influenza A (H3N2) strains.