Influenza has 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 above the epidemic threshold of 7.2 percent, CDC said. Nine of the 10 regions of the United States 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 U.S. region, the Southwest and California, had "normal" flu activity last week.
The vaccine against the flu strains that were forecast to predominate this year 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.
Experts recommend the vaccine for everyone over 6 months of age. Even if it does not prevent flu, it 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 against influenza A as well as influenza B. An A strain, called H3N2, predominates this season, though the B strain has caused about 20 percent of cases.
"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."
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 percentage of visits 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.
A Gallup Poll released on Friday found that 3.2 percent of Americans reported having the flu "yesterday" when they were asked the question in December, higher than in any December since Gallup began asking the question in 2008. That rate is more typical of February.
For its phone survey, Gallup asks 1,000 people each day whether they had the flu the day before, suggesting that the 3.2 percent may be an undercount: people with flu are less likely to respond to a pollster.
Hispanics were more likely than any other ethnic group to be stricken, Gallup found, with 9.2 reporting that they had the flu in December. Americans aged 30 to 44 were the age group most likely to report the flu in December. This is atypical, as reports of the flu generally decline with age, with those aged 18 to 29 usually reporting the most cases.
A total of 20 children have died from flu, up two from the previous week. That compares to 34 during the full 2011-2012 flu season, which was unusually mild, and 282 during the severe 2009-2010 season.
There are no data 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, which included the pandemic-flu year of 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.