The United States is facing an epidemic of influenza, with the share of flu-related deaths rising above the 7.2 percent threshold last week.
The Centers for Disease Control and Prevention (CDC) reported that nine of the 10 U.S. regions are experiencing "elevated" flu activity, with a small number of states exempt.
Of U.S. deaths last week, 7.3 percent were caused by pneumonia and flu — just above the "epidemic" threshold.
"The only thing that's predictable about flu is that it's unpredictable," said Friden. "Only time will tell us how long our season will last and how moderate or how severe this season will be in the end."
Experts recommend flu vaccines for everyone, particularly vulnerable populations like children and healthcare workers.
But some drugmakers reported shortages of flu shots and treatments this week.
Tamiflu maker Roche Holding AG said it was running low on the drug's liquid form. And Sanofi SA said it sold out of most dosages of its Fluzone vaccine, a common flu shot.
The CDC reported that this year's vaccine is 62 percent, or "moderately," effective in preventing the illness.