RALEIGH — The 2011-2012 flu season seems to be in its final weeks, but it’s hard to tell for sure, because – like winter itself this year – it barely happened.
The flu season usually builds in fall, hits a peak in late winter and wanes in spring. This time, it started late and never really picked up steam, according to figures from the state’s flu monitoring system.
The worst came in late February and early March, but it was hardly dramatic.
“The peak of activity is the lowest we’ve seen in at least a decade,” said Zack Moore, medical epidemiologist with the state Department of Health and Human Services.
The state keeps a running tally of people with flu-like illnesses – those with a fever and coughing or a sore throat – at 90 doctors’ offices, clinics and hospitals. At the peak of a bad flu season, perhaps 6 to 8 percent of the patients at these “sentinel” sites will have such an illness, Moore said; in a mild year, it’s perhaps 2 percent.
Cases are still occurring, but have trailed off now for several weeks. Unless there is a late-season resurgence, the tally for the worst week was 1.79 percent, the lowest on record since the reporting system was put in place in 2001.
Nationally, the picture was much the same, both for flu and weather. Many areas reported unusually mild flu seasons, and nationally the season didn’t even officially begin until February, the latest start in 29 years, according to the Centers for Disease Control and Prevention.
The fickle flu
Flu is notoriously fickle, with strains evolving or simply disappearing and new ones showing up. Experts say it’s probably impossible to figure out all the reasons one flu season turns out differently from another.
It’s likely, though, that the extraordinarily mild weather this winter was a factor in the weak flu season, said Dr. Sam Weir, an associate professor at UNC’s School of Medicine and co-director of the UNC Family Medicine Center.
“I don’t think there is any way to prove it, but it’s plausible, obviously, because the survivability of flu particles has been shown to be better in colder weather when the humidity is down,” Weir said.
Also, he said, milder winters mean people aren’t as likely to be cooped up together indoors, where the viruses spread more easily.
Other factors that are likely to have contributed to the weak season include improved inoculation rates in recent years, and flu strains that had appeared last year and the year before.
People who were vaccinated last year may have dodged flu this time even if they didn’t get vaccinated this year because they had residual resistance, Weir said. Also, those who contracted flu two years ago may have built up some natural resistance.
“This means that a larger-than-normal proportion of the population had some form of protection, whether by natural resistance or the vaccine,” Weir said.
Most flu deaths aren’t reported, and state health officials track only confirmed flu deaths – thought to be a fraction of the total. Often, there are several dozen in a season. So far, there have been just seven confirmed deaths in North Carolina attributed to flu since the season began last fall.
There is at least one sign the state won’t be as lucky next time.
Each year, flu experts monitoring strains emerging in the Southern Hemisphere and Asia make their best guess so a flu vaccine can be developed for the coming season, a cocktail that covers three varieties.
For next season’s vaccine they’re changing two of the three strains, said Dr. David Weber, a professor of epidemiology at UNC who has collaborated with the CDC on flu research.
“That may mean that we’ll be going back to higher rates again next season,” Weber said.