TORONTO - While a third wave of H1N1 influenza is unlikely this year, many Canadians over age 50 are still vulnerable to the virus and should get a flu shot, researchers say.
Their advice is based on findings from a study of B.C. residents that shows those aged 50 to 79 had the lowest rate of protective H1N1 antibodies of any age group following last year's pandemic.
"Where we see residual susceptibility at the highest rates is in those 50 to 79 years of age, where we measured only about 30 per cent had seroprotective antibody levels," said lead researcher Dr. Danuta Skowronski of the B.C. Centre for Disease Control.
Skowronski said adults in that age group are also at higher risk for complications if they get infected with H1N1, one of three flu strains incorporated in this year's seasonal flu shot. The other two are H3N2 influenza A Perth and influenza B Brisbane.
There may be several reasons why the rate of antibody protection was so low among the 50-79 age group, especially compared to children and teens, she said. The study found 70 per cent of those under age 20 had antibodies against H1N1 in post-pandemic testing.
"So if there was a lot of transmission going on in young children -- and we do typically think of school-age children as being the spreaders of influenza -- they probably had very high attack rates that contributed to their immunity," said Skowronski.
"And remember last year, (we were) primarily targeting the vaccine in the beginning to young children. So they were prioritized early to receive this ... vaccine. So that topped up the immunity that those kids were getting from already high infection rates."
The study, published in this week's issue of the Canadian Medical Association Journal, compared blood levels of antibodies against H1N1 influenza before and after the 2009 pandemic. They looked at blood samples from 1,127 people in B.C.'s Lower Mainland, aged nine months to 101 years old.
Samples collected before the pandemic indicated that less than 10 per cent of children and adults under age 70 had protective levels of H1N1 antibodies, whereas 77 per cent of people over age 80 had antibody protection.
The high proportion of the elderly with antibodies might be explained by "original antigenic sin," a phenomenon in which the immune system remembers exposure to a similar infection in childhood and calls up antibodies to fight off the new invader, Skowronski explained.
"So those over 90 years of age, they were exposed to viruses in 1918 up to 1930. Their first exposure was to a virus closely related to this swine pandemic H1N1 from 2009."
In follow-up testing after the fall 2009 immunization campaign, researchers found H1N1 antibodies in 70 per cent of those under 20, in 44 per cent of adults aged 20 to 49 and in 30 per cent of those 50 to 79. People 70 to 79 had the lowest rate of antibodies at 21 per cent, while those over 80 had higher rates.
"What we found was a dramatic change from pre-pandemic to the post-pandemic," said Skowronski.
Yet she and her fellow researchers are particularly puzzled by those aged 70 to 79, whose rate of H1N1 antibody protection didn't increase much from before and after the widespread influenza outbreak.
"It stuck at around 25 per cent," said Skowronski, who hopes researchers elsewhere in Canada will look at their populations to see if they have similar findings and try to figure out why.
"It may have something to do with original antigenic sin, that the first virus that they encountered was an H1N1 virus, but it was somewhat different ... So it may be that (their immune systems) kind of recognize it but not totally."
"I don't know the answer to that. Does it signal some kind of aberrant vaccine response or is it just something methodological in what we did?"
Dr. Mark Loeb, an infectious disease specialist at McMaster University, said the study represents a valuable tool for seeing what happened pre- and post-pandemic and it confirms what doctors pretty much expected.
"But I don't think this is necessarily the case for shifting policy," he said Monday from Hamilton. "I think the message should be that anyone who wants to avoid getting influenza should get a flu shot, generally speaking."
Indeed, public health officials encourage anyone over six months old to get vaccinated against the flu to prevent complications of the disease, which each year kills at least 4,000 Canadians.
H1N1 is probably not going to be the dominant strain circulating this flu season, Loeb said. "It's more likely going to be H3. So the more important question is what's the distribution of seasonal H3 (antibodies)?"
Skowronski also believes there will be a resurgence of H3N2 flu this season, and said there have already been sporadic cases in British Columbia and elsewhere in the country.
"So by focusing our immunization on older adults, especially those with chronic conditions, with this vaccine this year, we're going to get multiple benefits in terms of protecting against severe outcomes."
Also in this week's CMAJ, Drs. Donald Low and Allison McGeer of Toronto's Mount Sinai Hospital analyzed Canada's response to the H1N1 pandemic, saying that while mistakes were made, there were also valuable lessons learned that will help prepare for future influenza outbreaks.
"Last year's events clearly show that our current methods of vaccine production are too slow for an adequate response to a pandemic, that much of our planning for pandemic-related vaccination was incomplete, and that even the best-intentioned program can be undermined by unanticipated internal and external events," the infectious disease specialists write.
Despite planning, Canada did not adequately address risks to vulnerable populations such as pregnant women, aboriginal people and those in close contact with infected individuals, Low and McGeer said.
Learning how to clearly communicate risk to the public should be a priority before the next pandemic, they stressed.
