There is something distinctly missing from this year's flu season - the flu.
After a busy fall, when the pandemic H1N1 influenza virus infected thousands, dominated headlines and caused hysteria, it has dropped off the radar. But it's not only the pandemic strain that's disappeared, there doesn't seem to be much seasonal flu circulating either.
"In the past, you have peaks, hills, valleys. This is like the bottom of the Grand Canyon, and it's been going like this for weeks," said Donald Low, chief microbiologist at Toronto's Mount Sinai Hospital. "I would have thought that as soon as H1N1 dips, the vacuum would be filled with H3N2 and even seasonal H1N1. But nothing, nada."
The dearth of influenza, not just here, but in the United States as well, has doctors and health officials collectively scratching their heads. Has pandemic H1N1 replaced the seasonal viruses? And if so, has its early spread delayed - or perhaps even prevented - the normal flu season?
It wasn't too long ago that labs across the country churned out tests confirming the presence of one positive influenza specimen after another. Today, the Ontario public lab, for example, may be getting about 100 to 200 specimens a day for testing, but the tide has clearly turned. "It's been weeks since we've had a positive" for H1N1 or seasonal flu, Dr. Low said. Respiratory syncytial virus, a bug that causes respiratory tract infections, is the only thing spreading at the moment.
Dr. Low wondered if somehow the people exposed to H1N1 or vaccinated against it sustained a level of protection against other influenza viruses that diminishes their ability to take hold. "It's not what we consider great protection, and not enough that we would feel comfortable in saying that somebody is immune. But there's a lot we don't understand about protection," he added.
Indeed, flu viruses are as finicky and unpredictable as ever. Just like an unwelcome guest, they come and go as they please. Seasonal strains have been replaced by pandemic strains, and there is every reason to believe the pandemic H1N1 will cause illness next winter and the winter after that.
For now though, Michael Gardam, director of infectious diseases prevention and control at the Ontario Agency for Health Protection and Promotion, said the flu is in hiding. After a few years of wimpy influenza seasons, this would mark the wimpiest one yet, he said.
To some degree, the disappearance of H1N1 in Canada can be explained: Because about 45 per cent of Canadians have been vaccinated, and another 30 per cent were diagnosed with it and are thus immune, there are fewer people to infect, makingits spread that much more difficult. But explaining the absence of seasonal influenza gets tricky. Perhaps it's because of a mild winter, which prevents the flu from circulating as fiercely. Perhaps it's because the seasonal flu strains have run out of steam and enough people are immune to them.
Or, perhaps this flu season has behaved like other pandemic years with an early surge that then peters out.
One thing is certain: The peak of flu season has come and gone. It's unusual, but not out of the realm of possibility, to see flu resurface in March.
"There's still time. But that being said, every week that goes by with no flu, it gets harder to keep pushing that idea, because it's simply not out there," Dr. Gardam said.
While watching for influenza's next move, health experts are also focusing on the next season. There could be the possibility of H3N2 returning. But if only pandemic H1N1 resurfaces, it will be good news, because there is an effective vaccine to protect young people and pregnant women, who are among the most vulnerable, and it causes less morbidity and mortality than H3N2, which disproportionately affects the elderly.
Kevin Forward, chief of the division of microbiology at Capital Health in Halifax, said there haven't been enough pandemics of late to make a confident prediction regarding influenza's pattern.
"I would agree with the speculation that the pandemic flu is likely to evolve into a seasonal flu and that we will see it. Whether it's this year or next year remains to be seen," Dr. Forward said. "But there is enough in other parts of the world. The virus is still alive and spreading in other areas."
Dr. Forward said the lab in Nova Scotia has had a "very occasional positive test" for influenza. But, like other experts, he's knows better than to give up on this year's flu season entirely.
"It will turn up when it's darn good and ready," he said.
Caroline Alphonso