Why Do Influenza Virus Subtypes Die Out? A Hypothesis

When a new strain of flu takes hold, why does the old strain die out? Why doesn’t it continue making people sick every year? The authors of a new Opinion piece in mBio have an idea. They say the secret’s in the stalk.

Novel flu strains arise in those folks unlucky enough to get infected with two different strains at the same time. The two strains get together, share some genes, and have the potential to create a combination that human immune systems haven’t seen before. If it’s virulent, this hybrid flu virus can sweep through the population and create a pandemic. But what of the original strains? They seem to go away, but why? Why don’t we have seven or ten or 50 strains of flu in circulation at any given time?

For their part, Palese and Wang think the reason may be that the new strain retains a critical characteristic of the old strains: the stalks that holds up the hemagglutinin blobs on the surface of the virus. Antibodies to these stalks aren’t real potent, so they can’t keep you from getting infected with the flu virus, but they CAN recognize lots of different kinds of flu viruses, since the stalks are more conserved than many other parts of the virus.

Palese and Wang think of the old flu strain as collateral damage in the immunological battle against the novel virus. When a person confronts the new flu virus, these broadly-neutralizing anti-stalk antibodies are deployed to fight it, diminishing the severity of the novel virus, but also acting to eliminate the old virus. They say antibodies against another surface protein, viral neuraminidase, can act in much the same way, helping to eliminate old seasonal strains.