TORONTO - Another storm may be brewing for the coming flu season: A component of the seasonal flu shot may not be well matched to the circulating viruses, potentially setting up what's known as a vaccine mismatch.
Some samples of the emerging new strain of H3N2 viruses show a substantially reduced response to antibodies generated by the corresponding virus in the seasonal vaccine, raising the possibility of significantly reduced protection in some cases.
Vaccine mismatches are bad at the best of times. More people get sick during flu seasons with mismatches. But a seasonal flu vaccine mismatch coinciding with a flu pandemic? That is no one's idea of a good time.
Dr. Allison McGeer groaned when she heard a new H3N2 variant is circulating in some parts of the Southern Hemisphere.
"It's going to be a long winter. I know that already," said McGeer, an influenza expert and head of infection control at Toronto's Mount Sinai Hospital.
"It's not going to be pleasant because ... it's going to be one big long influenza season, from some time in September until next May."
The new variant has been seen on a number of continents, though it still remains a minority member of the H3N2 family, according to experts at the World Health Organization and the U.S. Centers for Disease Control in Atlanta.
With the demands the ongoing pandemic is placing on the WHO's laboratory network, researchers haven't yet had time to study whether the new variant is making up a growing percentage of H3N2 viruses, said Dr. Nancy Cox, director of the CDC's influenza division.
If they were, that would suggest the variant was on its way to becoming the dominant H3N2 virus and a vaccine mismatch would be on the cards.
Further clouding the issue is the fact that labs around the world haven't been submitting as many H3N2 viruses to the WHO network. There are simply fewer of them around.
"We haven't had that many H3N2 viruses to analyze because we've had such a flood of the novel H1N1 viruses because they're predominating," Cox said.
Dr. Wenqing Zhang of the WHO's global influenza program said many Southern Hemisphere countries are just coming into their regular flu seasons.
The next month or so will give the world a clearer picture as to whether the pandemic virus will crowd out the previous influenza A subtypes - making the composition of the seasonal vaccine less relevant - or whether one or both of the seasonal flu A virus families will continue to circulate.
And if H3N2 sticks around, the coming weeks will also offer a sign as to whether this new variant is likely to complicate the upcoming Northern Hemisphere flu season.
"While seemingly this variant H3 is emerging, we do not know to what extent it will be circulating," Zhang said by email from Geneva.
Cox said the WHO collaborating labs will be heightening surveillance for seasonal flu viruses in the lead-up to the WHO's strain selection meeting for the Southern Hemisphere seasonal vaccine, held in September.
One place they'll be checking is virus samples from nursing homes. H3N2 viruses prey on seniors and are behind many of the frequent flu outbreaks seen in long-term care facilities. For some reason, nursing home outbreaks are rarely caused by seasonal H1N1 viruses and so far the pandemic H1N1 has largely spared that population too.
"If we start to see outbreaks in those types of settings, this would increase our level of concern," Cox said. "We haven't seen that to date."
Because it takes months to make and ship flu vaccine, the viruses covered by the seasonal flu shot have to be chosen long in advance.
For the Northern Hemisphere vaccine, experts like Cox gather at the WHO in February to assess the viruses circulating and make their best estimates of which will be the major disease sources in the following winter.
For the 2009-2010 winter they chose an H3N2 virus called A/Brisbane/10 first spotted in 2007.
But within weeks of the decision hints emerged there was a new H3N2 variant, one sufficiently mutated ("drifted" in the language of flu) that it might be able to evade the vaccine.
Researchers in the influenza lab at the British Columbia Centre for Disease Control spotted it in early March as they were conducting late-season surveillance looking for just such viral evolution. In early May, they reported the finding on ProMed, an electronic bulletin board and mailing list which monitor infectious diseases outbreaks around the world.
Dr. Danuta Skowronski, an influenza expert at the BCCDC, said she is expecting this new variant to take over as the dominant H3N2 strain. And since the swine flu virus isn't infecting older adults much, it could leave a niche for the new H3 variant.
"So it's hard to know. We just have to prepare for that possibility either way," Skowronski said.
"The idea of having a mismatched drift strain circulating the same year that we also have swine influenza may mean that all segments of the population are affected by influenza one way or the other, whether it's the elderly with H3N2 and the young with H1N1."