Flu viruses mutate often and it is common to have to change at least one of the components of the vaccine from one year to the next in order to maximize the protection the vaccine can offer.
But detailed analysis of viruses collected from around the world suggest there haven't been any major changes that require an adjustment to the vaccine at this point, the experts concluded.
"What we're seeing is that the viruses really haven't moved (changed) antigenically and that the vaccine is performing as expected," explained Dr. Nancy Cox, head of the influenza division at the U.S. Centers for Disease Control and a member of the panel that advised the WHO on the flu vaccine composition.
Interestingly, the no-change recommendation means that the H1N1 virus in the 2014-15 vaccine will be the same one that has been used since the 2009 H1N1 pandemic — making the sixth year in a row that variant has been used in the vaccine.
Experts have been watching that virus, expecting that it will have to mutate soon to continue to be able to infect people, given how long the current variant has been spreading. But despite the fact that H1N1 was responsible for nearly all the flu illness in North America this winter, there are no signs yet the virus has mutated.
Dr. Danuta Skowronski, a flu expert at the British Columbia Centre for Disease Control, noted in a recent interview that historically H1N1 viruses have mutated — or drifted, in flu parlance — far less frequently than the other family of influenza A viruses, H3N2s.
"From 1990 to 2008, there have been five switches made to H1N1 vaccine components (in the vaccine) versus 11 for H3N2," said Skowronski, who was also in Geneva for the strain selection meeting.
"There were two H1N1 strains — but no H3N2 strains — that were retained as (vaccine) components for at least seven consecutive years in that period. And in fact one was retained for 11 consecutive years.
"So in fact H1N1 viruses in general tend to be more stable. Or at least they go through a slower pace of ... change, for whatever reason."
It takes months to make flu vaccine, so the strains that go into next winter's shot have to be chosen before the current flu season is over. Influenza researchers from around the world armed with reams of data gather at the WHO headquarters in Geneva twice a year for the so-called strain selection meeting — in February for the following Northern Hemisphere winter and in September for the next Southern Hemisphere flu season.
Telling vaccine manufacturers to use the same strains as a previous year will make their tasks simpler. Part of the art of making flu vaccine is coaxing viruses to grow well and deliver a good yield. Having worked with these viruses before should mean manufacturers that make flu vaccine have a bit of a leg up.
But an easy job this year could mean a particularly tough one might be down the road, Cox noted.
"Next time could be very challenging. Several different components could change," she said in an interview from Geneva.
Until recently, seasonal flu vaccines were all trivalent products designed to protect against three strains of flu viruses, two influenza As — H1N1 and H3N2 — and one of two families of influenza B viruses.
In recent years some manufacturers have started to make a four-component or quadrivalent vaccine which contains the two influenza As and the two B viruses.
In addition to news on the composition of next winter's flu vaccine, Thursday also saw an update on how well this year's vaccine is doing at protecting the people who got it.
The U.S. CDC released interim vaccine effectiveness estimates which suggest that vaccination reduced the risk of getting sick enough to need medical care by about 61 per cent this year on average across all age groups. The H1N1 component of the vaccine reduced the risk by 62 per cent on average, according to the U.S. data.
A Canadian study on the effectiveness of this year's flu vaccine released earlier this month showed slightly higher rates of protection — 71 per cent overall for the vaccine and 74 per cent for the H1N1 component.
It's not clear why the rates differ. Dr. Anne Schuchat, director of the CDC's national centre for immunization and respiratory diseases, said the differences were likely related to methodologic issues — how the studies were done — and probably did not signal that there was a real difference in how well the vaccine protected in Canada versus the United States.
Skowronski, who led the Canadian vaccine effectiveness study, said U.S. estimates are typically lower than Canadian ones. Cox said in general terms the estimates are largely similar: "It's absolutely the same ballpark."
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