Officials from the Centers for Disease Control and Prevention, speaking on a telephone news conference, again urged Americans to keep getting flu shots. At the same time, they emphasized that the shots are not infallible: a preliminary study rated this year’s vaccine as 62 percent effective, even though it is a good match for the most worrisome virus circulating. That corresponds to a rating of “moderately” effective — the vaccine typically ranges from 50 percent to 70 percent effective, they said.
Even though deaths stepped — barely — into epidemic territory for the first time last Saturday, the C.D.C. officials expressed no alarm, and said it was possible that new flu infections were peaking in some parts of the country. “Most of the country is seeing a lot of flu and that may continue for weeks,” said Dr. Thomas R. Frieden, the C.D.C.’s director.
New outpatient cases — a measure based on what percentage of doctor visits were for colds or flu — dropped off slightly from the previous week, to 4 percent from 6 percent. The trend was more pronounced in the South, where this year’s season began.
Dr. Frieden cautioned that the new flu figures could be aberrations because they were gathered as the holiday season was ending. Few people schedule routine checkups then, so the percentage of visits for severe illness can be pushed artificially high for a week or two, then inevitably drop.
Deaths from pneumonia and the flu, a wavy curve that is low in summer and high in winter, typically touch the epidemic level for one or two weeks every flu season. How bad a season is depends on how high the deaths climb for how long.
So far this season, 20 children with confirmed flu tests have died, but that is presumably lower than the actual number of deaths because not all children are tested and not all such deaths are reported. How many adults die will not be estimated until after the season ends, said Dr. Joseph Bresee, the chief of prevention and epidemiology for the C.D.C.’s flu branch. Epidemiologists count how many death certificates are filed in a flu year, compare the number with normal years, and estimate what percentage were probably flu-related.
Many people are getting ill this year because the country is also having widespread outbreaks of two diseases with overlapping symptoms, norovirus and whooping cough, and the normal winter surge in common colds. Flu shots have no effect on any of those.
Spot shortages of vaccines have been reported, and there will not be enough for all Americans, since the industry has made and shipped only about 130 million doses. But officials said they would be pleased if 50 percent of Americans got shots; in a typical year, 37 percent do.
Dr. Bresee said that this year’s epidemic resembles that of 2003-4, which also began early, was dominated by an H3N2 strain and killed more Americans than usual.
Nevertheless, more Americans now routinely get flu shots than did then, and doctors are much quicker to prescribe Tamiflu and Relenza, drugs that can lessen a flu’s severity if taken early.
The C.D.C.’s vaccine effectiveness study bore out the point of view of a report released last year by the University of Minnesota’s Center for Infectious Disease Research and Policy. It said that the shot’s effectiveness had been “overpromoted and overhyped,” said Michael T. Osterholm, the center’s director.
Although the report supported getting flu shots, it said that new vaccines offering lifelong protection against all flu strains, instead of annual partial protection against a mix-and-match set, must be created.
“But there’s no appetite to fund that research,” Dr. Osterholm said in an interview Friday.
“To get a vaccine across the ‘Valley of Death’ is likely to cost $1 billion,” he added, referring to the huge clinical trials that would be needed to approve a new type of vaccine. “No government has put more than $100 million into any candidate, and the private sector has no appetite for it because there’s not enough return on investment.”
At the same time, he praised the C.D.C. for measuring vaccine effectiveness in midseason.
“We’re the only ones in the world who have data like that,” he said.
“Vaccine effectiveness” is a very different metric from vaccine-virus match, which is done in a lab. Vaccine efficacy is measured by interviewing hundreds of sick or recovering patients who had positive flu tests and asking whether and when they had received shots.
Only people sick enough to visit doctors get flu tests, said Thomas Skinner, a C.D.C. spokesman, so the metric means the shot “reduces by 62 percent your chance of getting a flu so bad that you have to go to a doctor or hospital.”
During the telephone news conference Friday, Dr. Frieden repeatedly described the vaccine as “far from perfect, but by far the best tool we have to prevent influenza.”
Most vaccinations given in childhood for threats like measles and diphtheria are 90 percent effective or better. But flu viruses mutate so fast that they must be remade annually. Scientists are trying to develop vaccines that target bits of the virus that appear to stay constant, like the stem of the hemagglutinin spike that lets the virus break into lung cells.
During the 2009 swine flu pandemic, many elderly Americans had natural protection, presumably from flus they caught in the 1930s or ’40s.
“Think about that,” Dr. Osterholm said. “Even though they were old, they were still protected. We’ve got to figure out how to capture that kind of immunity — which current vaccines do not.”
At Friday’s news conference, Dr. Bresee acknowledged the difficulties, saying: “If I had the perfect answer as to how to make a better flu vaccine, I’d probably get a Nobel Prize.”
