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So far, 29 states are reporting high levels of "influenza-like illness," according to the CDC. Levels are 10 times higher in some cities than this time last year.
This year's early and potentially severe flu season has created shortages of flu vaccines and of the children's formulation of the leading treatment, according to their manufacturers.
The number of people suffering from the flu in the United States is almost 10 times higher than last year in some cities, according to health authorities
The largest flu vaccine provider in the United States, Sanofi SA, said on Thursday that it had sold out of four of its six formulations of its Fluzone seasonal flu vaccine due to the unanticipated late-season demand for vaccines.
Roche, the manufacturer of Tamiflu, told Reuters late Wednesday that it had a shortage of the drug's liquid formulation. Tamiflu is used mainly to slow down or stop the symptoms of the flu in children already infected with the virus.
Roche said the company had informed wholesalers and distributors in recent weeks that they would face temporary delays in shipments. Pharmacists can create a substitute oral formulation by dissolving Tamiflu capsules into a sweet liquid, according to Tara Iannuccillo, spokeswoman for Roche's Genentech unit, which makes Tamiflu.
Health authorities said flu season usually starts in December, but began in November this year. The particular flu strain going around — H3N2 — has a reputation for causing fairly severe illness, especially among the elderly.
Hospitals around the country are running out of space and some have had to turn people away. A public health emergency was declared in Boston on Wednesday. The city is working with health care centers to offer free flu vaccines and also hopes to set up places where people can get vaccinated. The city said there had been four flu-related deaths, all elderly residents, since the unofficial start of the flu season on Oct. 1.
Mayor Thomas Menino said there had been about 700 confirmed cases of the flu in Boston so far this season, compared with 70 all of last season.
Massachusetts was one of 29 states reporting high levels of "influenza-like illness," according to the most recent weekly flu advisory issued by the Centers for Disease Control and Prevention.
The CDC said the proportion of people visiting health care providers with flu-like symptoms climbed from 2.8 percent to 5.6 percent in four weeks. By contrast, the rate peaked at only 2.2 percent during the relatively mild 2011-2012 flu season.
The estimated rate of flu-related hospitalizations in the United States is 8.1 per 100,000 people, which is high for this time of year, according to Dr. Joe Bresee, chief of the epidemiology and prevention branch of the CDC's influenza division. The agency's next advisory will be issued Friday.
"This is the worst flu season we've seen since 2009, and people should take the threat of flu seriously," Menino said in a statement. "This is not only a health concern, but also an economic concern for families, and I'm urging residents to get vaccinated if they haven't already."
Menino also urged people to stay home from work or school if they are sick.