Strains of a common subtype of influenza virus, H3N2, have almost universally acquired a mutation that effectively blocks antibodies from binding to a key viral protein.

The results have implications for flu vaccine design, according to the researchers. Current flu vaccines, which are «seasonal vaccines» designed to protect against recently circulating flu strains, induce antibody responses mostly against a different viral protein called hemagglutinin.

The new mutation, described in the study published online June 29 in PLOS Pathogens, was first detected in the 2014-2015 flu season in some H3N2 flu strains, and evidently is so good at boosting flu’s ability to spread that it is now present in virtually all circulating H3N2 strains. Recent flu seasons, in which H3N2 strains have featured prominently, have been relatively severe compared to historical averages.

The mutation alters a viral protein called neuraminidase, and the researchers found in their study that this alteration paradoxically reduces the ability of flu virus to replicate in a type of human nasal cell that it normally infects. However, the researchers also found evidence that the mutation more than compensates for this deficit by setting up a physical barrier that hinders antibodies from binding to neuraminidase.

«These findings tell us that flu vaccines focusing on the hemagglutinin protein are leaving the virus openings to evolve and evade other types of immunity,» says study senior author Andrew Pekosz, PhD, professor and vice chair of the Department of Molecular Microbiology and Immunology at the Bloomberg School.

Every year, influenza viruses sicken millions of people around the world, killing several hundred thousands. The diversity of flu strains and their ability to mutate rapidly — two strains infecting the same host can even swap genes — have made flu viruses an especially difficult target for vaccine designers. Although scientists are working towards a universal vaccine that will protect long-term against most flu variants, current flu vaccines are designed to protect against only a short list of recently circulating strains. Any mutation that occurs in these circulating strains and appears to improve their ability to spread is naturally of interest to flu virologists.

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Materials provided by Johns Hopkins University Bloomberg School of Public Health. Note: Content may be edited for style and length.