Only 12% of adults think they had the flu this past winter, fewer than the 15% to 21% who thought they had the flu in other winters since 2004.
We emphasize "think" because it is widely believed that not everyone who has flu-like symptoms actually has the flu.
However, there is no evidence that flu shots should get the credit for the decline in the number of people with the flu.
The 39% of adults who had flu shots is almost the same as those who had them in two previous winters (40% in 2008-2009, and 36% in 2007-2008). Furthermore, for the third year running - the proportion of adults who believe they got the flu was the same (12%) among those who did and did not get flu shot.
These are some of the findings of a new Harris Poll® survey of 2,755 U.S. adults, surveyed online between April 12 and 19, 2010 by Harris Interactive.
This is a complicated subject and there are several reasons for caution in interpreting these findings - for example, it would be erroneous to conclude that the flu vaccines used last winter had no effect.
However, it is probably safe to conclude that the flu vaccines used to protect people over the last three winters were less effective than the vaccines used to protect people in the winters of 2006-07, 2004-05, and 2003-04.
In each of those winters, people who received shots were significantly less likely (especially in 2004-05) to get the flu (or to believe they got it) than were those who did not have flu shots.
Reasons for Caution
There are two reasons to be careful when analyzing these data.
1) One is that the diagnosis, and particularly self-diagnosis, of the flu is not very reliable. Other infections can produce flu-like symptoms. This raises the question of whether some people who report having had flu shots and then having the flu may have been mistaken.
This year's survey, similar to previous years, finds that about three-quarters of these people are "certain" they had the flu (72%) and spent one or more days in bed (76%). However, only just two in five (39%) visited a doctor who diagnosed the flu.
2) Furthermore, the absence of a correlation does not necessarily mean that the flu shots had no effect, because those who are more likely to get the flu may also be more likely to get flu shots. The Harris Poll suggests that this happened.
Fully 68% of people aged 65 and over - a high risk group - had flu shots, and 46% of people aged 50-64 did so. Far fewer adults under 50 did, which could explain why people over 50 were less likely to get the flu than younger people.
Other findings of this Harris Poll include:
* Men and women were equally likely to have had flu shots (38% for men, 39% for women) and equally likely to believe they had the flu (12% for both men and women).
* People under 40 were more likely than people over 40 to report having had the flu. This may be a result of the higher incidence of flu shots among older people or the higher levels of immunity built up over time by older people.
* African-Americans (6%) are much less likely than Whites (12%) and Hispanics (19%) to report having had the flu, although similar proportions of these three groups received flu shots.
* Most (66%) of those who report having had the flu this winter believe they had a "regular flu." Only 13% believe they had the H1N1 flu virus. More than a fifth (22%) were not sure.
* The proportion of adults who think they will have a flu shot next winter is the same as those who had one this last winter (39%), and they are mostly the same people. Only 3% of those who had a flu shot say they will not get one for this coming winter.
This research is no substitute for the kind of double-blind clinical trials that are used to assess the efficacy and safety of new drugs. However, we believe that the year to year trends are meaningful and important.
We conclude therefore that the vaccines used in the last three winters (2007/08, 2008/09 and 2009/10) were less effective than the vaccines used in three previous winters where these questions were asked in Harris Polls (2003/04, 2004/05, 2006/07).
Scientists have reported that developing flu vaccines is particularly difficult because flu viruses tend to mutate rapidly. This may be one explanation for the relatively poor results of the vaccines used before the last three winters.
This Harris Poll was conducted online within the United States between April 12 and 19, 2010 among 2,755 adults (aged 18 and over). 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.
Where appropriate, these data were also weighted to reflect the composition of the adult online population. Propensity score weighting was also used to adjust for respondents' propensity to be online.
All sample surveys and polls, whether or not they use probability sampling, are subject to multiple sources of error which are most often not possible to quantify or estimate, including sampling error, coverage error, error associated with nonresponse, error associated with question wording and response options, and post-survey weighting and adjustments.
Therefore, Harris Interactive avoids the words "margin of error" as they are misleading. All that can be calculated are different possible sampling errors with different probabilities for pure, unweighted, random samples with 100% response rates. These are only theoretical because no published polls come close to this ideal.
Respondents for this survey were selected from among those who have agreed to participate in Harris Interactive surveys. The data have been weighted to reflect the composition of the adult population.
Because the sample is based on those who agreed to participate in the Harris Interactive panel, no estimates of theoretical sampling error can be calculated.
These statements conform to the principles of disclosure of the National Council on Public Polls.
The results of this Harris Poll may not be used in advertising, marketing or promotion without the prior written permission of Harris Interactive.
Q805, 810, 815, 820, 825, 830
The Harris Poll ® #64, May 14, 2010
By Humphrey Taylor, Chairman, The Harris Poll, Harris Interactive
About Harris Interactive
For more information, please visit www.harrisinteractive.com .
New York, N.Y. - 14 May 2010