The study of 1,818 hospital workers in Colorado, a state hit hard early in the flu season, showed the vaccine had “no or low effectiveness against influenza-like illness.”
The CDC cautioned that the study was not conclusive and that the agency had not changed its recommendations based on the findings. One weakness of the study, the agency explained on its Web site, was that the researchers did not test ill hospital workers to confirm that they had the flu, instead all “flu-like illnesses” were lumped together.
The study showed that people who were vaccinated against the flu came down with these flu-like illnesses — meaning they had a fever, cough, and/or sore throat — at the same rate as people who were not vaccinated.
Experts at the CDC began this study, which is the first of several designed to test the vaccine’s effectiveness, because of concern that this year’s protection against the virus may have been less effective than usual. The vaccine is formulated each year in February or March and did not include the Fujian strain of influenza, a new, mutated strain that turned out to be the most common type of flu in the United States and several other countries this year.
Although the vaccine did not include the Fujian strain, it did contain the related Panama strain, which experts hoped would provide at least some protection. This early study suggests that was not the case.
Of the 1,000 people in the study vaccinated before Nov. 1, about 15 percent came down with a flu-like illness. Of the 402 participants who were not vaccinated, just under 17 percent got such an illness.
The CDC noted that generally when studies include all flu-like illnesses, the vaccination appears to be less effective than if the researchers actually test to see if people have the flu.
CDC spokesman Tom Skinner said the study shows only part of the story.
“This is the first of a number of studies that we are going to do to try to answer the question, ‘Did this year’s vaccine offer protection against flu?’ This study in and of itself does not answer this question,” he said in a telephone interview.
Thursday’s findings come after many people searched long and hard to track down a dosage of the vaccine. There were shortages in some places and the U.S. government bought 625,000 additional vaccine doses from makers Chiron and Aventis.
Meanwhile, this season’s epidemic appears to be subsiding. During the week ending Jan. 10, only 2.8 percent of hospital visits were for influenza-like illnesses, compared to 5.5 percent the previous week.