Second Strain Might Have Caused Some Severe Cases In Mexico, Investigators Say

Second Strain Might Have Caused Some Severe Cases In Mexico, Investigators Say

Via NPR

A new discovery from Canada raises the question — has a new mutation in an ordinary flu virus been causing some of the recent respiratory disease hospitalizations and deaths in Mexico?

It’s too soon to tell, but scientists in Vancouver are wondering. They’ve found two mutations in H3N2, a regular seasonal flu virus that’s been circulating in North America since last fall. (The swine flu virus is a type of H1N1.)

The mutations affect a protein called hemagglutinin that sits on the outer coat of Type A flu viruses. That’s what the “H” stands for in H1N1, H3N2 and other Type A viruses. (The “N” stands for neuraminidase, another surface protein.)

The new version of H3N2 has shown up in a number of nursing home patients in British Columbia, though not yet in the general community there. And this week the Canadian researchers spotted it when they did a complete genetic analysis of a flu virus that sickened a Canadian traveler who had just returned from Mexico.

This raises the possibility that the traveler became infected in Mexico, says Dr. Robert Brunham, chief of the British Columbia Centre for Disease Control. If so, the H3N2 virus circulating in Mexico may have the same two mutations as those being found in British Columbia patients who haven’t been to Mexico .

“In British Columbia, the H3N2 virus causes more severe cases of flu than H1N1 does,” Brunham says. “So we wonder if some of the severe cases in Mexico may have been caused by the variant H3N2 virus.”

There’s another potential implication of the Canadian finding.

Public health officials watch flu viruses closely for mutations that might make new variants of viruses less susceptible to flu vaccines. A strain called H3N2/Brisbane is one of the three components in this year’s seasonal flu vaccine — and it’s also in the vaccine now being made for next winter’s flu season.

So far the new variation of H3N2 hasn’t crossed the threshold of concern for scientists about the effectiveness of next season’s vaccine. But, “it only takes one more mutation” to cross that threshold, Brunham says.

But nobody can say yet whether the new H3N2 mutant has been circulating widely in Mexico. That’s something researchers really want to find out ASAP.

British Columbian health officials have tested a number of people who have returned from Mexico recently suffering from cold or flu symptoms, along with hundreds of non-travelers. Most didn’t have any sort of flu. Among the 10 percent who did test positive for flu, they were split 50-50 between H1N1 and H3N2 types.

However, health officials have so far only done close genetic analysis on one Mexico traveler. So they can’t say how many of the others have the new version of H3N2.

8:48 AM ET | 05-06-2009

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