The Centers for Disease Control and Prevention said for the week ending Feb. 25, the number of respiratory specimens testing positive for flu increased to 18.4 percent, compared with 14.4 percent the week before.
For the entire United States, the level of influenza-like illnesses is relatively low, but the Northeast, the Northern Midwest and the central Midwest were experiencing higher levels.
The Centers for Infectious Disease Research & Policy at the University of Minnesota reported nationally, the percentage of doctors visits for influenza-like illnesses held steady at 1.9 percent, putting it below the national baseline of 2.4 percent.
H3N2 is the most commonly detected strain of influenza, but the CDC said a growing proportion of pandemic 2009 H1N1 viruses, especially in Western and Southwestern states.
The European Centre for Disease Prevention and Control said Europe's flu season -- similar to the flu season in the United States -- has shown a later start this year versus past seasons, with the number of countries reporting high- or medium-intensity activity last week holding steady compared with the previous week.
However, Bulgaria and Italy reported decreasing flu indicators for the past three weeks in a row, suggesting they are the first European countries where flu has peaked.