Officials at the Centers for Disease Control and Prevention in Atlanta said the percentage of respiratory specimens that tested positive for flu the week ending Feb. 4 jumped from 7.6 percent the week before to 10.5 percent passing the 10-percent mark that health officials typically use to gauge when the season has started.
California became the first state to cite widespread flu transmission this year.
Nationally, the percentage of doctor visits for flu-like illness was 1.7 percent, below baseline, but regional baselines were exceeded in two parts of the country: Iowa, Kansas, Missouri and Nebraska; and the northwest, including Alaska, Idaho, Oregon and Washington, the Centers for Infectious Disease Research & Policy at the University of Minnesota reported.
Although H3N2 viruses are still dominant, the CDC reported a rising number of 2009 H1N1 detections over the past several weeks, especially in south-central and southwestern states.
The Pan American Health Organization said Mexico reported 2,815 influenza infections and 58 deaths, with about 90 percent of the cases and 93 percent of the deaths due to the 2009 H1N1 virus.