Two small clusters of drug resistance don't a public health crisis make. But experts worry they could signal the development of a Tamiflu-resistant pandemic virus with the ability to spread from person to person -- at least under certain circumstances.
Health officials are investigating two unrelated clusters of hospital patients -- four patients in North Carolina, five in Wales who've been infected with swine flu viruses resistant to the mainstay antiviral drug Tamiflu.
All of the involved patients reportedly had weakened immune systems. That may have enabled the pandemic virus to replicate in their systems more freely. If these immuno-compromised patients had been given Tamiflu, that combination of factors may have led the virus to develop a point mutation conferring resistance against the drug.
The worry is that if a Tamiflu-resistant virus "learns" to transmit from one person to another, it might break out of hospitalized populations to the community at large.
That's what happened with that other H1N1 flu virus -- the one that causes seasonal flu. It's almost 100 percent resistant to Tamiflu and its cousin Zanamivir.
Seasonal H1N1 viruses carry the same mutation that has popped up so far in all the pandemic H1N1 resistant viruses, according to the World Health Organization.
The WHO recently published a review of 39 Tamiflu-resistant cases -- the number it had counted as of October 22. Seven of them were immuno-compromised patients, like the recent clusters.
Now, it's not a foregone conclusion that the pandemic virus will become widely resistant to Tamiflu. It hasn't yet, and presumably it's had lots of opportunity. But when the CDC's Anne Schuchat was asked Friday about the Welsh cluster she said: "That's the kind of report we take seriously."
She was less concerned about another swine flu mutation that popped up in three Norwegian flu victims. http://www.who.int/csr/disease/swineflu/notes/briefing_20091120/en/index.html
That mutation seems to enable the virus to glom onto cells deeper in the lung. Two of the Norwegian patients died and the third is critically ill.
"This mutation has been seen sporadically here and there around the world -- sometimes in people who had mild disease and sometimes in fatal disease," Schuchat says. "It's an important finding for virologists, but I don't think it yet has the public health implications we would worry about."