At the time of year when lots of Americans are usually coughing, sneezing, aching and feverish with the flu, doctor's offices and hospitals across the U.S. have been surprisingly quiet so far.
February is usually the month when U.S. flu cases peak, and more people become sick than in any other month. But the U.S. Centers for Disease Control and Prevention says this year's flu season has only just begun.
"This is the latest start to a flu season in the past 29 years," said Dr. Joseph Bresee, chief of the influenza division at the CDC's Epidemiology and Prevention Branch in a teleconference.
Officials typically declare a start to the flu season once the number of respiratory specimens testing positive for flu rises above 10 percent, which usually happens by late December, sometimes as early as October.
But this year, the numbers didn't hit the 10 percent mark until early February. Last week, about 15 percent tested positive for flu, up from almost 13 percent the week before.
Bresee said it's unclear exactly why the flu has been so sluggish this year, but a few theories are floating around. For one, the warmer weather in most parts of the country may make it harder for flu viruses, which thrive in cold, dry climates, to survive. More pleasant weather could also mean people are spending less time indoors in close quarters, transmitting viruses to one another.
Also, the strains of the virus are relatively the same this year as in 2011. Most of this season's flu cases have been caused by H3N2 and H1N1. The H1N1 virus has been circulating for the past three years, said Dr. Jon Abramson, professor and chairman of pediatrics at Wake Forest Medical School.
"The virus is essentially unchanged," Abramson said. "More people have been exposed to it so fewer people are susceptible to it."
Bresee also noted that the lower numbers of flu cases point to the success of flu vaccines. More people were vaccinated by November 2011 than by November 2010, although it's hard to count exactly how many have gotten a flu shot, he said.
But even though the flu season has been less virulent so far, it's likely that it will ramp up in the coming months.
According to the CDC's weekly flu report, all 50 states have reported some flu activity, and California and Colorado reported widespread flu activity. Although the numbers of flu-related doctor's visits, hospitalizations and deaths are lower than this time last year, three children have died from the flu so far.
Abramson said it's not too late to get a flu shot, which the CDC recommends for everyone older than 6 months old.
"If you haven't been vaccinated yet, you should be," he said. "It is likely there will be some peak in the flu in March."