Government epidemiologists insist that many things go into making a flu season nasty or mild, early or late.
But a great deal of accumulating research points a feverish finger at long-term weather patterns influencing both the timing and severity of flu season.
Of course, flu season traditionally comes in the winter, and the assumption is that whatever vile viral stew is going around spreads more easily when people spend more time together indoors.
In one ongoing experiment led by a Columbia University public health and climate researcher, forecasters have combined trends in Google searches for flu topics with weather-pattern observations and forecasts to accurately predict some past flu seasons, and perhaps the current outbreak. Their first real-time test in New York City was accurate in predicting infection peaks in specific areas up to seven weeks in advance.
The same researcher, Jeffrey Shaman, reported with colleagues last year that the four worst flu outbreaks of the past century -- from 1918-19 to the 2009 swine flu epidemic -- closely followed the emergence of La Nina, a phenomenon characterized by extreme cold-water currents around the equator in the Pacific.
The 1918 pandemic infected more than 500 million people and killed an estimated 20 million to 50 million in several waves. Recent reports suggest more than 20 percent of Americans were infected by the novel virus that first struck in the spring of 2009, and hit children and young adults particularly hard over several waves.
La Nina events tend to disrupt migratory bird patterns, making it easier for avian flu strains to mix and spread among birds and into other species, the researchers said.
Conversely, there's evidence that a strong El Nino -- with unusually warm ocean temperatures in the equatorial Pacific -- aided the 1918 pandemic, researchers at Texas A&M University reported in 2009 based on work supported by the National Oceanic and Atmospheric Administration. The weather shift shut down the monsoons in India and had other global effects that contributed to the flu's deadly effects.
The latest flu-weather connection was published Monday by researchers at Arizona State University. After studying U.S. flu outbreaks from the 1997-98 season until now, they concluded that warm winters are usually followed by heavy flu seasons.
They found several other instances of seasons that matched the current outbreak. Flu 2012-13 struck early and is hitting hard -- harder than usual -- because relatively few people got sick during last year's mild winter and mild flu season.
The Centers for Disease Control and Prevention said last season set a record for the lowest and shortest peak in cases in which people sought medical attention for the flu. The peak lasted one week; in most seasons, doctors report a peak in flu-related visits stretching from three to 13 weeks.
A mild flu season like last year leaves a greater percentage of the population vulnerable because they haven't acquired immunity either from natural infection or by vaccination if the flu ramps up really early the next time.
The researchers write, in the online journal PLOS Currents: Influenza, that warm winters are likely to become more common if the global rise in temperatures continues, making it more likely that flu will hit earlier and harder when colder conditions do return.
(Contact Scripps health and science writer Lee Bowman at BowmanL@shns.com .)