Spring may seem just around the corner but much of the country remains mired in the throes of flu season. Northern states are experiencing a bitterly cold winter, but weather, it turns out, is a poor predictor for determining America’s Sickest Cities. Many of these sickly cities are clustered in the South.

Gallery: America’s 20 Sickest Cities, From Miami to New York

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“Flu usually starts in Florida, Texas, and then it goes up the southern coast usually and it’ll start to go out to the Midwest and out to the Northeast and then it’ll get out to the West Coast, except for Hawaii—Hawaii is its own little animal,” says Glenna Rohlfing, director of clinical operations for SDI Health, an analytics firm. “We don’t know why. Some will say it’s because Southern schools go back to school earlier, they start earlier than the Northeast schools, everybody has their speculation.”

To determine America’s sickest cities, The Daily Beast combined four data points that give a clear snapshot of which cities are suffering from the sniffles (weighting is in parenthesis):

SDI Health Ranking (30 percent): SDI does its own ranking based on the percentage of positive influenza tests in 75 large U.S. markets. Our list of America’s 20 Sickest Cities was first pared down by the 20 markets that, according to SDI’s nationwide network of health-care data providers, have experienced the highest percentage of positive influenza tests from the week ending September 4 through February 19. Flu season runs from October through May, according to the Centers for Disease Control and Prevention.

Statewide Vaccination Coverage (10 percent): The percentage of people age 65 and younger that have seasonal influenza vaccination coverage as of January 2010, based on estimates from the CDC. Better vaccination coverage indicates a less-sick city.

Influenza and pneumonia deaths per 100,000 people (30 percent): Based on 2010-2011 flu season data from the CDC.

Estimated flu activity, remainder of the flu season (30 percent): Using the three most recent years of data from Google Flu Trends, The Daily Beast estimates the percentage by which flu activity is expected to increase or decrease from March to May for each city—the remainder of the flu season. Google Flu Trends is based on influenza-related search data collected by Google and updated in near real time. It tends to accurately reflect the official positive influenza test data released later by the CDC.

Ranking and research by Clark Merrefield