Last Year's Flu Vaccine May Have Been Less Effective Than Vaccines Used In Previous Years

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Flu Vaccine

A new Harris Poll suggests that the flu vaccine used before last winter may have been less effective than the vaccines used in some previous years.

The adults who had flu shots before last winter were only 24 percent less likely to get the flu than those who were not vaccinated. The differences were larger in two previous Harris Polls on the same topic.(1) In the winters of 2003/2004 and 2004/2005 adults who were vaccinated were 33 percent and 43 percent less likely to have gotten the flu than those who were not.

These are some of the results of a nationwide Harris Poll of 2,563 U.S. adults surveyed online by Harris Interactive(R) between April 10 and 16, 2007.

All of these results should be treated with some caution. People's memories of whether they had flu shots may not be completely accurate. This is not a double-blind clinical trial, which is the gold standard for measuring the effectiveness of drugs. Furthermore, experts on the flu say that it is easy to confuse it with other infections, so some people who believe that they had the flu may not actually have had it. However, a large 71 percent majority of those who received flu shots and who believe they had the flu say they are certain they had it; but only just over a third (39%) visited a doctor who diagnosed the flu. The techniques used in this year's survey are the same as those used in previous years.

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The main findings of this survey include:

- Approximately one-third (35%) of all adults claim to have had a flu shot before the winter of 2006/2007. This includes a much higher proportion (73%) of people aged 65 and over, who are regarded as more seriously at risk from the flu if they catch it;

- Fifteen percent of all adults believe they had the flu, somewhat below the 18 percent and 21 percent who believe they had the flu in the winters of 2003/2004 (18%) and 2004/2005 (21%);

- The proportion of adults who had received flu shots who believe they subsequently caught the flu (13%) was only somewhat lower than the proportion of those who had not received flu shots who subsequently caught the flu (17%).

The difference between these numbers (13% and 17%) is smaller than the differences found after the winters of 2003/2004 (14% and 21%) and 2004/2005 (13% and 23%). However the survey contains some good news. Fewer people (15%) caught the flu this last winter than in the two previous winters we studied (18% and 21%).

This Harris Poll(R) was conducted online within the United States between April 10 and 16, 2007 among a nationwide cross section of 2,563 (aged 18 and over) of whom 899 got a flu shot before the winter of 2006/2007. Figures for age, sex, race/ethnicity, education, region and household income were weighted where necessary to bring them into line with their actual proportions in the population. Propensity score weighting was also used to adjust for respondents' propensity to be online.

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