Health officials continue to urge residents to get their flu shots as half of the United States reports a widespread outbreak.
Flu cases have been reported in every state, Puerto Rico and the District of Columbia, the U.S. Centers for Disease Control and Prevention said. Regional outbreaks have also been reported in 20 other states.
Studies have suggested that seasonal influenza viruses have a seasonal pattern; influenza viruses circulate at higher levels in cold weather and at lower levels in warm weather, but it is a subject that needs further investigation, Dr. Michael Jhung of the CDC said.
So far during the 2013-14 season, 2009 H1N1 influenza viruses have been most common to circulate and cause illness. The 2009 H1N1 influenza virus, the so-called swine flu, was first identified in 2009, when it emerged to cause an influenza pandemic.
"Since then, it has circulated worldwide as a seasonal flu virus, although this is the first year since 2009 that it has been the predominant strain," Jhung, a medical officer for the Surveillance and Outbreak Response Team in the Influenza Division of the National Center for Immunization and Respiratory Diseases of the CDC, said.
Six flu-related deaths have occurred in Pennsylvania, where there is a widespread outbreak of the flu, Pennsylvania Health Department Spokeswoman Holli Senior said.
All 67 Pennsylvania counties have reported flu cases, 90 percent of which are of the H1N1 variety.
Since the 2010-11 season, the flu vaccine started to cover the N1H1 strain.
"It's certainly not too late to get vaccinated," Senior said. "A flu shot is the best way to prevent the flu and protect you and your loved ones."
It takes about two weeks for the vaccine to build up full immunity, she said.
There are three classes of factors that may contribute to the influenza season in the U.S., Jhung said.
"The three classes are: one, factors related to host resistance to infection (seasonal variation in the body's immune response to infection with influenza viruses); two, factors related to environmental survival of influenza virus (seasonal variation in ambient temperature and relative humidity that impact survival of influenza virus outside hosts); and three, factors related to seasonal variation in host behavior (spending more time indoors during cold winter months, close contact of susceptible children in schools during fall and winter months)," he said.
Soon there may be a flu forecast, just like the daily weather and pollen count forecasts.
Jeffrey Shaman, assistant professor at the Department of Environmental Health Sciences for Columbia University, developed a forecast model to predict influenza's spread up to seven weeks in advance.
The model is predicated on the rate of influenza outbreaks being specific to each region and associated with absolute humidity levels, Shaman told AccuWeather.com in December.
The degree to which ambient temperature and relative humidity contribute is not completely understood, Jhung said.
"Influenza virus shows increased survival in environments with lower ambient temperatures and lower relative humidity (compared with higher temperature and higher humidity), and studies suggest that influenza virus transmission can be facilitated by cold, dry conditions," he said.
Widespread influenza activity is ongoing in Alabama, Alaska, Arkansas, Colorado, Connecticut, Georgia, Idaho, Illinois, Indiana, Kansas, Louisiana, Massachusetts, Minnesota, Nebraska, Nevada, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Texas, Utah, Virginia, Washington and Wyoming.
Regional influenza activity was reported in California, Florida, Iowa, Kentucky, Maine, Maryland, Michigan, Mississippi, Missouri, Montana, New Hampshire, New Jersey, New Mexico, North Dakota, Ohio, South Carolina, South Dakota, Tennessee, West Virginia and Wisconsin.
"Although it is too early to compare the duration or severity of the current season to previous seasons, increases in influenza activity are expected to continue in the coming weeks, and most influenza seasons typically show peak activity in January or February," Jhung said.