TORONTO - When you think of a flu pandemic, the images that come to mind are of people sick and people dying.
But influenza experts quietly admit there may be a silver lining -- or several -- to the H1N1 pandemic that erupted this year. Not just in the event itself, which was milder than feared, but also in the viral legacy it may leave.
In the wake of this pandemic, flu vaccine could be easier to make or could cover more targets. A tricky problem of drug resistance could disappear. And the toll influenza takes on the elderly could conceivably ease, at least for awhile.
Before going too far down What-If Road, however, it's important to note that predicting influenza's path is a mug's game. The longer people study it, the less likely they are to try to guess what influenza viruses may do.
"I don't know - right now everything's a possibility as far as I'm concerned," Dr. Michael Osterholm, director of the University of Minnesota's Center for Infectious Diseases Research and Policy, cautions when asked about what the flu landscape might look like in the aftermath of this pandemic.
Still, even experts who share that understanding are thinking about some possibilities.
Their optimism is in large part fuelled by a phenomenon known as viral replacement which has been seen in previous pandemics, or at least the three that have been studied using modern laboratory techniques. In simple terms, during the pandemics of the 20th century - 1918, 1957 and 1968 - the new virus snuffed out its viral predecessor.
If the same thing occurs as a result of the pandemic of 2009, the world might actually one day look back fondly on the swine flu virus that caught us all by surprise last spring, some suggest.
"If this pandemic virus were to replace seasonal strains, either H3(N2) or H1(N1), that may be a blessing in disguise," says Dr. Danuta Skowronski, an influenza epidemiologist at the British Columbia Centre for Disease Control.
To grasp the significance of what might be afoot, it's helpful to have some flu basics.
Influenza viruses are divided into three large families, A, B and C, though C viruses are thought to bit players when it comes to human illness.
Pandemics can only be triggered by influenza A viruses. And historically there was only ever one subtype of influenza A around at a time. But that changed in 1977 when H1N1 viruses, which had stopped circulating 11 years earlier, mysteriously re-emerged. (It is widely believed the virus "escaped" from a Russian lab as a result of a research accident.)
Since then, there have been two flu A viruses circulating, seasonal H1N1 and H3N2. Annual flu shots target both A viruses plus one of two families of influenza B viruses.
Influenza B causes a fair amount of human illness. And earlier this year there was some debate about making a four-component or quadrivalent vaccine to include both B families. But there has been hesitancy because making the trivalent vaccine every year is challenging enough.
If the pandemic H1N1 gets rid of both H3N2 and seasonal H1N1, vaccine manufacturers would only need to include one influenza A component - the 2009 H1N1 - in seasonal flu shots.
They could make a bivalent - two component - vaccine, which would be easier to produce. Or they could stick with a trivalent shot, but have it cover one A and both Bs, making the shot more protective.
Getting rid of both of the previous seasonal A viruses also appeals from another point of view. H3N2 is a nasty virus, one which takes a heavy toll on the elderly. No one in public health would miss H3N2 if it disappeared.
"It would be the most amazing thing," says Dr. Allison McGeer, a flu expert at Toronto's Mount Sinai Hospital.
"Because a the great majority of nursing home outbreaks are (caused by) H3N2. You get rid of 80 per cent of influenza outbreaks in nursing homes - (it would) be brilliant."
Dr. Anthony Mounts, a flu expert with the World Health Organization, says since this pandemic started people have gone back and studied the patterns of H1N1 and H3N2 seasons. When H1N1 viruses predominate, younger people are generally hit harder; during H3N2 seasons, as McGeer observes, the worst of the illness occurs in the elderly.
"Why that is, I don't think anybody really understands," he says.
But children and adults respond better to flu vaccine than do seniors, whose immune systems are breaking down with age. So if the burden of influenza shifts down the age spectrum, the primary tool available to fight it - vaccine - could be targeted to people who get more benefit from it.
And younger people are less likely to die of flu than seniors, in whom a bout of flu can be the proverbial final straw.
"That might be actually the silver lining, is that this is something that's less deadly than H3 and maybe something that we can do more about," says Mounts, who, like Osterholm, is quick to warn "it's all conjecture at this point."
The WHO's top flu expert injects a note of caution of his own.
Dr. Keiji Fukuda points out the pandemic caused by H3N2 was the mildest of the three in the last century. Its behaviour as a pandemic virus did not foreshadow what was to come.
"Do we know that this H1N1 virus is going to always be like it is now? The chances are: Probably not," Fukuda says.
"I mean, it could stay mild all the time, but I think the lessons from H3N2 is that something which starts out and looks relatively mild in fact can become something which becomes more severe over time."
As it is, many flu experts think we may be stuck with H3N2 for awhile yet. They are not convinced the pandemic virus can oust it as well as seasonal H1N1.
Dr. Arnold Monto, of the University of Michigan, notes H3N2 viruses are still circulating in pockets of Southeast Asia and in the tropics. He won't venture to guess whether they will die out there too, or if those regions will serve as a reservoir for resurgent H3N2 activity.
"Flu is un-pre-dict-able," he says, stringing out the word for emphasis.
Dr. Nancy Cox isn't convinced H3N2 is going away. But the head of the influenza division of the U.S. Centers for Disease Control would be happy with a one-for-one exchange, with the pandemic H1N1 replacing the seasonal virus of the same name.
That's because seasonal H1N1 viruses are resistant to oseltamivir (Tamiflu), the main drug used to fight flu.
The pandemic H1N1 viruses are susceptible to Tamiflu, though they are resistant to two older flu drugs, amantadine and rimantadine. Those two drugs aren't widely used anymore because resistance to them develops easily.
Swapping viruses that are immune to Tamiflu for ones the drug works against would be a bargain, Cox suggests. "Getting rid of resistance in circulating H1N1 viruses would be a real silver lining."
