NPR Health Editor Joe Neel diagrams how the new H1N1 virus might reassort into something more dangerous.
More than half the flu viruses infecting people across the nation are the new swine flu strain, and that has health officials fretting.
The latest worry is that the new H1N1 virus will pick up resistance to the only two drugs effective against it -- oseltamivir (Tamiflu) and zanamavir (Relenza) -- by mixing it up with the wrong crowd, i.e. viruses that are already resistant.
It would work like this:
One of the older seasonal flu viruses that's still around, also in the H1N1 family, is already resistant to these antiviral drugs. If somebody got simultaneously infected with one of the older viruses and the new swine flu, the viruses could exchange genes within that person's cells.
This is called reassortment. Flu viruses are good at it, because their genes are segmented into neat mix-and-match packets.
The resulting "reassortant" virus could be drug-resistant, says Dr. Anne Schuchat of the Centers for Disease Control and Prevention.
"This is a really important issue," she said at Tuesday's CDC swine flu media briefing. " If those two strains reassorted and we got a new strain that was a combination of the two, we would hate to see this novel strain become resistant through that mechanism."
That would leave doctors with no effective drug against the novel strain. All those Tamiflu stockpiles would be worthless.
The longer both these H1N1 flu viruses are circulating throughout the country, the greater that chance favors the emergence of such a resistant virus. It can happen quickly. Flu experts were surprised at how fast the old, seasonal H1N1 acquired resistance to the mainstay antiviral drugs.