Over the past four weeks, some hospitals in parts of Eastern Canada have observed a significant surge in the number of patients admitted to hospital with influenza or complications (pneumonia, bacterial infections) of it. Sadly, some patients have died despite best efforts and timely medical care.

Some hospital intensive care units have been at full capacity such that critically ill patients who arrive there have required transfer elsewhere to receive ongoing care.

Some Toronto hospitals almost suspended elective surgeries to create capacity to handle this surge in admitted patients. Nursing home outbreaks have been identified, and have been closed to admissions and transfers, compounding the “gridlock” problem further.

All of these developments have been newsworthy, underscoring the fact that seasonal flu remains a serious disease that has significant impact on the health care system.

a) Why are some years worse than others?
“Bad” flu years are usually the result of a drift (a subtle mutation) of the previous year’s flu virus strain. Because the strain has changed, less of the population is immune, whether previously vaccinated or previously infected. The consequence is a higher rate of community disease in the previously healthy, and serious disease in the vulnerable. Long-term care facilities see outbreaks as health care providers and visitors unwittingly transmit the virus to the vulnerable. The vulnerable are, figuratively speaking, “at the edge of a cliff”. Infection with the flu can “push them over this cliff” and into the emergency room, hospital beds, and intensive care unit beds.

The vaccine match (decided months in advance of flu season) is fair to poor in years of drifted strains, and more disease occurs despite the appropriate annual fall ritual of vaccinating the vulnerable and the healthy (e.g. health care providers, those who share a household with he vulnerable) in their midst.

b) Is this year exceptional?
Yes, that an unchanged seasonal (H3) flu strain is causing a lot of illness is exceptional. The virus in the dominant strain this year has not drifted, and the vaccine match with the circulating strain is good. In my clinical experience over the past few weeks, this year is reminiscent of prior “bad” flu years like 1997 and 2003. (http://www.nejm.org/doi/full/10.1056/NEJMp0904819)

The reasons for a bad flu year featuring an unchanged virus will be a subject of investigation. Conclusions that a general disinterest in vaccination is the cause of this year’s challenges may not be the entire story. We need more data on vaccination uptake rates in Canada and the U.S. before coming to this conclusion.

One unusual scientific observation is that the larger second H1N1 pandemic wave in the Northern Hemisphere (fall 2009) resulted in something called “strain replacement”: seasonal (H3) influenza strain was temporarily “squeezed out” by the H1N1 pandemic strain until the last few weeks. Therefore, the vulnerable did not have contact with the unchanged seasonal flu virus until recently. The one-year hiatus of circulation H3 virus may have impaired the protection of the vulnerable, despite recent vaccination. The vulnerable who skipped the vaccine altogether, would have lost whatever immunity they did have from infection or vaccination in the remote past, and may be even worse off as a result of strain replacement. (This is why it is still very important for the vulnerable to receive this year’s vaccine.)

c) If it is an exceptional year, just how bad is it?
As compared with prior seasonal flu years, it’s still within the realm of normal. The 2003-2004 (Fujian drifted strain) and 2007-2008 (California drifted strain) years were also remarkable for having caused havoc. Hospitals were at capacity at that time too. Many vulnerable people died. We keep hearing that adage: “when it comes to the flu, expect the unexpected”. But given our knowledge of the current strain, there is no reason to believe it will behave remarkably differently locally as compared with elsewhere. Indeed local variations are common – we saw that with H1N1. The Canadian (http://origin.phac-aspc.gc.ca/fluwatch/) and US (http://www.cdc.gov/flu/weekly/) surveillance data speak volumes, and allow for the most objective comparison with previous flu years. So far, none of the indicators suggest that something ominous is occurring this year.

d) So what’s in store, and what can be done?
Flu waves do not occur in lockstep; different areas are affected at different times. Each local flu outbreak lasts four to six weeks, and minimal disease due to that strain is seen locally after that for the rest of the flu season. Paradoxically, by the time the serious illness and death counts are mounting locally, the peak of the outbreak has been reached, and the number of new infections is falling. Despite valiant efforts, it’s too late to change the trajectory at that point.

For areas that are yet to see widespread flu activity, there is still time to gear up with aggressive vaccination campaigns especially directed at those who most need it. And doctors need to think of the flu when treating the most ill, and promptly prescribe antiviral medications, in some cases before a flu test becomes available. ICU networks need to be prepared for a transient surge in the number of sick patients. And areas that have already passed their peaks can assist their more seriously afflicted neighbours by accepting patients in transfer.

Bottom Line:

Our collective memories are short. The recent H1N1 pandemic (June 2009 – August 2010) nothwithstanding, the preceding two seasonal flu years were relatively quiet.

Now we are experiencing a flu season that is exceptional from a scientific perspective, but that remains within the realm of normal from an impact perspective.

And the pandemic, though it caused considerable community illness and killed more young people than we typically see, spared the elderly such that hospitals saw fewer admitted patients during the pandemic than we are seeing now.

More on this topic: Read Dr. Marla Shapiro's blog, It's Not Too Late to Get a Flu Shot!