The flu vaccine is far from 100 percent effective, but researchers have calculated that over the past six years it has prevented millions of illnesses and tens of thousands of hospitalizations.
The analysis, from the Centers for Disease Control and Prevention, was published in the journal PLoS One. Using national surveillance data, the researchers estimated the number of cases and hospitalizations in each flu season from 2005-6 through 2010-11. Then, accounting for year-to-year variations in vaccine efficacy, vaccine coverage and disease occurrence, they calculated the number of cases and hospitalizations that were avoided by vaccination.
The numbers varied by year and by age group, but over the six seasons, they found that about 13.6 million illnesses, 5.8 million medical visits and 112,900 influenza-related hospitalizations were prevented by using the vaccine. The largest impact was in 2010-11, when there were large increases in coverage, which the researchers attribute at least in part to the severity of the 2009 pandemic of H1N1 influenza, or swine flu.
“The vaccine’s not perfect,” said an author of the study, Carrie Reed, an epidemiologist with the C.D.C., “but some protection is better than none. Up to about 18 percent of cases were actually averted by our vaccination program, which is a sizable number.”
A future analysis will use similar techniques to determine the number of deaths avoided by use of the vaccine.