A year ago the H1N1 flu pandemic reached the United States, setting off a wave of vaccinations to protect those most at risk.
Thousands of people died, including more than 60 in Minnesota, most under age 65. But the seasonal flu was virtually non-existent in the state. As a result, few elderly people died this year from seasonal flu.
Of the hundreds of influenza virus samples tested by the Minnesota Department of Health this year, Kris Ehresmann, the state's top researcher, can count on one hand the number of A or B flu strains that were the seasonal variety.
"Some years we have hundreds, 500 cases, 300 cases reported to the department for testing," said Ehresmann, director of the department's Infectious Disease Epidemiology, Prevention and Control Division. "And so to say that we have four or five or even 10 in a season is really unusual."
This has happened before. During the pandemic flu outbreaks of 1957 and 1968, very little seasonal flu circulated. Researchers don't know why that's sometimes the case. Some speculate that pandemic strains out-compete the seasonal varieties of flu.
Others suspect that children are the group most likely to pass flu to the elderly, and if they're widely infected with a pandemic virus, there's less opportunity for them to spread regular seasonal flu to their grandparents. But both theories have some flaws.
Whatever the reason, it appears that the state's elderly residents, who are typically the most vulnerable to seasonal flu, had a very good year.
"It's one of the best seasons we've had in the sense of very few cases in nursing homes, very few outbreaks, almost none," said Dr. Dwight Townes, medical director for four long-term care facilities in the Twin Cities metro area.
"People had some upper respiratory infections, some colds. And there is always some pneumonia, especially in the winter in nursing homes," Townes said. "But as far as I know, and certainly in my homes, none of it was diagnosed as H1N1 or as seasonal influenza."
That observation is shared by Darrell Shreve, vice president for health policy at Aging Services of Minnesota -- the state's largest association of long-term care providers. His organization has over 1,000 members.
Shreve said no members on his association called this year asking whether they should report a resident's illness or death to the health department, as is required.
"I haven't had those calls this year at all," he said. "It was a quiet year."
It's too early to calculate exactly how much lower the flu death rate might be among the state's elderly population. That information won't come out until the end of the year, when the health department reports on the number of death certificates that have listed flu or pneumonia as a cause of death. Bacterial pneumonias are often a complication related to influenza.
In the past decade Minnesota has averaged about 800 flu and pneumonia deaths each year.
Presumably that number could be lower by several hundred this year. But that potentially good news is tempered by the higher than usual number of flu deaths among young people.
The H1N1 pandemic strain of flu killed more than 60 Minnesotans and the vast majority of them were under the age of 65.
"Years of life lost, or the number of years difference between average life expectancy and the time of the death, is up much, much higher this year than we'd see in a normal flu year," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
"We're well up to three to four times higher, the number of years of life lost," Osterholm said. "So total deaths are down. But they're down in the elderly population. They're way up in the younger population."
In contrast to flu deaths that occur later in life, these early deaths represent a significant economic loss for many of the victims' families and for the economy of the state, Osterholm said. So what may look like a mild to moderate flu season on paper is actually quite severe.
Public health officials expect that the H1N1 flu virus will become the dominant strain of flu circulating the globe. If that's the case, flu mortality statistics could be altered for years to come, making an unusual season like this a more regular occurrence.