CHICAGO More than three-quarters of Americans who got this season's flu shot could get the virus anyway, given a mismatch between the flu strains covered by the shot and those actually causing illness in people, U.S. officials said.
An interim CDC report found the shot was only 23 percent effective overall, a performance about in line with what the U.S. Centers for Disease Control and Prevention predicted last year, experts said. At the time, CDC warned that the predominant flu virus, influenza A (H3N2), had "drifted" or changed genetically since the shot was made.
Effectiveness varies widely by age, working best in young, healthy people and least well in the elderly, a pattern reflected in the report released Thursday.
It showed effectiveness against H3N2 viruses was highest - at 26 percent - among children aged 6 months to 17 years. It was roughly 12 percent effective in people 18-49 and 14 percent effective in those 50 and older, but those estimates were not statistically significant because there was too little data at this point in the flu season.
"These estimates of vaccine efficacy are doleful, and they are entirely consistent with what the CDC told us they were likely to be based on the studies in the lab," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville.
Since the CDC started doing flu vaccine studies in the 2004-2005 flu season, overall effectiveness has ranged from 10 percent to 60 percent.
The results reflect limitations in the current shots, in which flu strains for the Northern Hemisphere are selected in late February of the prior year, giving manufacturers roughly six months to make the vaccines from scratch before the start of the flu season in August.
Schaffner said this year's predictions of which virus would be dominant were fairly accurate, in that the shot contains H3N2, a subtype associated with more severe flu seasons. The problem was that the circulating version of H3N2 mutated just enough to reduce vaccine effectiveness.
CDC epidemiologist Brendan Flannery said H3 viruses can change from year to year, but "this is a pretty major one."
"There are some years when the virus drifts and the vaccine still provides pretty good protection. This is not one of those years," he said.
CDC still recommends vaccination because the virus could shift again before the flu season ends. For those who become sick, CDC recommends prompt treatment with antivirals such as Roche's Tamiflu.
(Reporting by Julie Steenhuysen; Editing by Christian Plumb)