# 5092
In September of 2009, news of an (at that time) unpublished Canadian study began to surface that suggested that those who had received a seasonal flu shot the previous year were more susceptible to the new pandemic virus than those who hadn’t.
Helen Branswell, science and medical reporter for the Canadian Press, was among the first to report on it (see Branswell On The Canadian Flu Shot Controversy).
This bombshell – which began to be known as `the Canadian problem’, sent shockwaves through public health circles. Many agencies were just days away from starting up their seasonal flu vaccination campaigns as they waited for the arrival of the H1N1 pandemic vaccine expected later in the fall.
Suddenly, there was genuine concern that maybe . . . just maybe . . . with a pandemic virus on the way, that rolling out the seasonal vaccine was the wrong thing to do.
The CDC and the World Health Organization both scrambled to look at their available data, and stated that they could find no correlation between the seasonal vax and susceptibility to the pandemic flu . . . but that they would continue to look.
Meanwhile, with concerns rising, a number of Canadian Provinces halted or announced delays in rolling out the seasonal flu shot, even though the study had yet to be published (see Ontario Adjusts Vaccination Plan).
October saw a number of new reports and studies that failed to corroborate the (still unpublished) findings, including a study published in the BMJ (British Medical Journal) that suggested exactly the opposite - that getting the seasonal flu vaccination may be slightly protective against the swine flu (see When Studies Collide).
Which, admittedly, ran contrary to what we’d heard previously from the CDC, who maintained that the seasonal vaccine was not expected to offer any protection against the novel H1N1 swine flu virus.
By November, with no compelling corroboration of the `Canadian Problem’, Canada’s National Advisory Committee on Immunization (NACI) came out in favor of resuming seasonal flu jabs (see NACI: Canada Should Resume Seasonal Flu Vaccinations).
The controversy wasn’t over, however.
In April of this year these Canadian studies were finally published by PLoS Medicine. Writing for CIDRAP, Maryn McKenna detailed their findings.
New Canadian studies suggest seasonal flu shot increased H1N1 risk
Maryn McKenna
Contributing Writer
Apr 6, 2010 (CIDRAP News) – Despite a rapidly launched range of studies, investigators in Canada are still unable to say—or to rule out—whether receiving a seasonal flu vaccination in the 2008-09 season made it more likely that Canadians would become ill from 2009 pandemic H1N1 flu.
Other studies continued to fail to show any correlation, leaving us with a bit of a mystery on our hands.
Why were these Canadian findings so different than all the others?
Which bring us to today’s story regarding a hypothesis that appeared in yesterday’s Eurosurveillance journal, suggesting a mechanism that might explain the `Canadian Problem’.
Eurosurveillance, Volume 15, Issue 47, 25 November 2010
Perspectives
H Kelly , S Barry, K Laurie, G Mercer
You’ll probably want to read the entire paper, but for those not inclined to wade through the whole article, I’ll attempt to summarize their hypothesis.
Unlike the Canadian researchers, Australian scientists could find no increased susceptibility to the pandemic H1N1 virus among those who had been vaccinated the previous year against seasonal flu.
The difference between the two findings, they suggest, comes from three separate factors:
- A theory regarding temporary immunity following any influenza infection
- The timing of the arrival of the pandemic virus in Canada
- And the protective effects of seasonal flu vaccination against seasonal - but not pandemic - flu.
We’ll take these one at a time.
It has been theorized that infection by any influenza (or perhaps, any respiratory) virus ramps up the body’s immune system for weeks or even months after the illness has passed, making that person temporarily less susceptible to infection by another virus.
Researchers, looking back at the infection patterns from the 1918 and 1957 pandemics, have used this theory to explain why pandemics come in waves.
And given that each year we usually see two A strains of influenza, a B strain, and a veritable rogues gallery non-influenza respiratory viruses circulating, this may also help explain why we all don’t endure non-stop ILI’s every winter.
Which bring us to the timing element.
In Canada, the first wave of the virus arrived on the heels of the 2008-2009 seasonal flu epidemic, which had peaked only 3 months earlier.
Australia, however, was nearing the end of their summer, and the peak of their flu season had occurred a full 9 months before.
If the temporary immunity theory is correct (`if’ being the operative word), Canadians who had contracted seasonal flu earlier in the year, may still have carried some generic immunity against infection.
Australians, on the other hand, saw the pandemic virus arrive long after any such temporary protective benefits would have decayed.
The third element is the protective benefits of the seasonal flu vaccine.
The 2008-2009 vaccine was reasonably protective against seasonal flu, but offered little or no protection against the novel H1N1 virus.
Since those that eschewed the seasonal vaccine were more likely to catch the flu, it increased their opportunity to develop the (hypothesized) temporary generic immunity discussed above.
The authors suggest that those who took the vaccine, lacking the `temporary protection’ from a recent bout with the flu, may have appeared to be more susceptible to the pandemic virus.
It’s an attractive solution, and many of the elements do seem to fit.
But to make it work, you have to accept the temporary immunity hypothesis as being valid - and while gaining acceptance – it hasn’t been fully proven.
And if this explanation is correct, you would also expect to find a similar pattern in other vaccinated regions of the northern hemisphere where novel H1N1 arrived shortly after the peak of their flu season.
So far, that hasn’t been demonstrated.
But good science takes time, which means that more research will be needed on several fronts before this theory can be accepted as a resolution to the `Canadian Problem’.
