Influenza has officially reached epidemic proportions in the United States, with 7.3 percent of deaths last week caused by pneumonia and the flu, the U.S. Centers for Disease Control and Prevention said on Friday.

That is just above the epidemic threshold of 7.2 percent. Nine of the 10 U.S. regions had "elevated" flu activity, confirming that seasonal flu has spread across the country and reached high levels several weeks before the usual time of late January or February. The other region, the Southwest and California, had "normal" flu activity last week.

This year's flu vaccine is 62 percent effective, scientists reported on Friday in the CDC's weekly publication.

That is considered "moderate" effectiveness and means that almost four in 10 people who receive the vaccine and are exposed to the virus will nevertheless become infected. It is line with the effectiveness of previous years' flu vaccines, which typically range from 50 percent to 70 percent, Dr. Joseph Bresee, chief of the CDC's influenza division, said in a telephone conference call on Friday.

Tens of thousands of Americans die every year from flu, even in non-epidemic years. The CDC in its report did not give a total number of deaths due to flu.

Experts recommend the vaccine for everyone over 6 months of age. Even if it does not prevent flu, immunization can reduce the severity of the illness, preventing pneumonia and other life-threatening results of flu.

Public health authorities were correct in their forecast of which flu strains would emerge this season and therefore what vaccine to make: one that contains two strains of influenza A and one strain of influenza B. An A strain, called H3N2, predominates this season, though the B strain has caused about 20 percent of cases.

About 10 percent of cases have been caused by a B strain that is not in the vaccine, which for technical reasons "has space for only three strains," CDC Director Dr. Tom Frieden said.

"We have a good vaccine but not a great vaccine," Dr. Arnold Monto of the University of Michigan, a co-author of the vaccine-effectiveness study, said in an interview. "Every year we see vaccine failures."

The vaccine is less effective in the frail elderly, in people receiving chemotherapy for cancer, and in people taking oral steroids. That's because their immune systems have been weakened and are often unable to produce an effective number of antibodies in response to the vaccine.

"That's the opposite of what we would wish," Frieden said. "The people most susceptible (to flu) are less likely to get the benefits of the vaccine."

In its weekly flu update on Friday, the CDC reported that 24 states and New York City experienced "high activity" in flu-like illnesses last week. In 16 states flu activity was moderate, while 10 states reported low or minimal flu activity.

The 24 states reporting high flu activity is down from 29 the previous week, raising hopes that flu may have peaked in some regions, particularly the Southeast, and that a season that began early will also end early. It typically starts in December, peaks in January or February and peters out by late March or early April.

The percentage of visits to healthcare providers last week for flu-like illness, 4.3 percent, is comparable to that during the 2007-2008 flu season, which was characterized as "moderately severe" but which peaked some two months later. During the 2009 H1N1 "swine" flu pandemic, 7.7 percent of visits were for flu-like illness.

Compared to last year's mild flu season, doctors' offices and hospitals have been overwhelmed with flu cases, with patients waiting through the night to be seen in emergency departments.

In Illinois, 24 hospitals struggling to cope with the flood of flu cases had to turn away people arriving in the emergency department, while in Pennsylvania, the Lehigh Valley Hospital outside Allentown has set up a tent for people who arrive with less-severe flu.

In Boston, flu cases are 10 times higher than they were last year, causing Mayor Thomas Menino to declare a public health emergency on Wednesday.

A total of 20 children have now died from this season's flu, up two from the previous week, the CDC said. That compares to 34 during the full 2011-2012 flu season, which was unusually mild, and 282 during the severe 2009-2010 season.

No data is available on how many children have received flu shots this season, but in the past it has fallen far short of public health experts' recommendation that everyone 6 months and older be vaccinated.

From 2004 to 2009, fewer than 45 percent of children were vaccinated against the flu, researchers led by Dr. Katherine Poehling of Wake Forest School of Medicine in Winston-Salem, North Carolina, reported this week in the journal Pediatrics.

According to the most recent CDC data, 37 percent of Americans - 112 million people - had received the flu vaccine as of mid-November. Of the 135 million doses produced this year, 128 million have been distributed to doctors' offices, drug stores, clinics and other facilities.

That has resulted in spot shortages, said CDC's Bresee. "You may have to call a few places" before finding one with vaccine, "but it should be available."