Newer flu vaccines may have been less effective than vaccines used in previous years

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.

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%).

TABLE 1 THOSE WHO HAD FLU SHOTS AND THOSE WHO GOT THE FLU THIS WINTER "Thinking back to this winter just ending ... Did you get the flu this winter? Did you have a flu vaccine shot before this winter?" Base: All Adults March March April 2004 2005 2007 % % % Percentage of all adults who had a flu shot before this winter 35 27 35 Percentage of all adults who got the flu this winter 18 21 15 TABLE 2 HOW MANY OF THOSE WHO GOT OR DID NOT HAVE FLU SHOTS GOT THE FLU? "Thinking back to this winter just ending ... Did you get the flu this winter? Did you have a flu vaccine shot before this winter?" Base: All Adults March March April 2004 2005 2007 % % % Percentage of all adults who had a flu shot who got the flu 14 13 13 Percentage of all adults who did not have flu shots who got the flu 21 23 17 How much less likely were people with flu shots to get the flu than those who were not vaccinated? 33 43 24 TABLE 3 THE EXPERIENCES OF THOSE WHO BELIEVE THEY GOT THE FLU AFTER HAVING HAD A FLU SHOT "Did you spend one or more days in bed with the flu?"

"Did you visit a doctor who diagnosed the flu?" "How certain are you that you got the flu, and that it was not just a cough or

a cold?" Base: All adults who had a flu shot and believe they had the flu March March April 2004 2005 2007 % % % Spent one or more days in bed 79 85 74 Visited a doctor who diagnosed flu 51 53 39 Certain I got the flu 80 82 71 TABLE 4 DEMOGRAPHICS OF THOSE WHO HAD FLU SHOTS AND OF THOSE WHO GOT THE FLU "Did you get the flu this winter? Did you have a flu vaccine shot before this winter?" Base: All adults Got the Flu Had a Flu Shot % % All Adults 15 35 Sex Male 13 36 Female 17 34 Age 18-24 20 26 25-29 24 15 30-39 19 21 40-49 16 22 50-65 10 43 65+ 9 73 Race/Ethnicity White 14 36 African-American 15 33 Hispanic 24 32 Methodology

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.

All surveys are subject to several sources of error. These include: sampling error (because only a sample of a population is interviewed); measurement error due to question wording and/or question order, deliberately or unintentionally inaccurate responses, nonresponse (including refusals), interviewer effects (when live interviewers are used) and weighting.

With one exception (sampling error) the magnitude of the errors that result cannot be estimated. There is, therefore, no way to calculate a finite "margin of error" for any survey and the use of these words should be avoided.

With pure probability samples, with 100 percent response rates, it is possible to calculate the probability that the sampling error (but not other sources of error) is not greater than some number. With a pure probability sample of 2,563 adults one could say with a ninety-five percent probability that the overall results have a sampling error of +/- two percentage points. However that does not take other sources of error into account. This online survey is not based on a probability sample and therefore no theoretical sampling error can be calculated.

These statements conform to the principles of disclosure of the National Council on Public Polls.

J29951 Q 756, 760, 765, 770 The Harris Poll(R) #37, May 2, 2007 By Humphrey Taylor, chairman of The Harris Poll(R), Harris Interactive. About Harris Interactive

http://www.harrisinteractive.com/