PRINCETON, NJ -- In light of the ongoing spread of swine flu, a review of data collected as part of the Gallup-Healthways Well-Being Index reveals the extent to which Americans generally report flu illness. On any given day, a baseline of between about 1% and 4% of the adult population reports being sick with flu, including about 2.1% on an average day so far this April. Older Americans may be more likely to suffer serious consequences if they contract flu, but the Gallup-Healthways data show that self-reports of having the flu become less prevalent as the population ages.
The Gallup-Healthways Well-Being Index surveys a random sample of about 1,000 adults each day, or roughly 30,000 interviews a month, on a variety of topics related to health and wellbeing. One of the questions asked each day is, "Were you sick with any of the following yesterday?" The question specifies four illnesses: the flu, a cold, a headache, and allergies.
The accompanying graph shows the month-by-month percentages in reported flu illness from January 2008 through April of this year (which includes interviews conducted April 1-27). There is a clear cyclical pattern of incidence of flu as measured by these self-reports, with the highest reports coming in the late fall and winter months, and the lowest reports in the summer.
Given the current swine flu situation, these data provide a baseline for monitoring any increase in these self-reports that may occur. Between 2.1% and 2.2% of American adults reported having the flu in April of either 2008 or 2009. Last May, the number dropped to 1.6%.
It is unlikely that the spread of swine flu in the United States will be so massive that it would move the percentages in these reports in and of itself. Each percentage of the adult population represents about 2.2 million people, so even tens of thousands of cases of swine flu would not by themselves have a major impact on national samples.
Still, widespread publicity about a disease such as swine flu no doubt will increase Americans' focus on their health and symptoms. Thus, it is possible that those who in the past may have ignored symptoms will find themselves rushing to the doctor to have their symptoms diagnosed, and this could cause the self-reported incidence of flu to increase as a secondary byproduct of the swine flu situation. There has, however, been no increase in self-reports of flu in the last several days. Gallup will continue to report these numbers in the days ahead.
Many people confuse flu symptoms with common-cold symptoms. Colds, at least as reported by average Americans, are significantly more prevalent than flu.
The percentage of the adult population that reports having been "sick with a cold" on any given day throughout the year ranges from a little more than 2% to nearly 11%. Here again, there is apparently predictable cyclicality. Colds are most commonly reported in the late fall and winter, and drop off significantly in the summer months.
There is a significant inverse correlation between age and reports of having been "sick with the flu yesterday" in the Gallup-Healthways data.
These data are an aggregate of more than 110,000 interviews conducted so far in 2009, from Jan. 2 through April 27. As can be seen in the accompanying graph, younger adults are most likely to report being sick with the flu, and the percentage drops fairly steadily with age. All of Gallup's interviews are conducted with adults aged 18 and older, so incidence of flu among children and among teenagers aged 13 to 17 would not be reflected.
It is important to remember that these data reflect being "sick with the flu" and do not indicate mortality as a result of flu. It's likely that the consequences of having the flu are much more significant among older people, even if the prevalence of the disease itself is lower. Additionally, older Americans who are so sick with flu that they are hospitalized would not be available for interviewing.
Gallup will continue to track self-reports over the coming days and weeks and will also be asking a special series of questions related to how, if at all, people are changing their behaviors as a results of the swine flu situation. Visit Gallup.com for results.
Results are based on telephone interviews with 355,334 national adults, aged 18 and older, conducted Jan. 2-Dec. 30, 2008, and 115,738 interviews conducted Jan. 2,-April 27, 2009. For results based on the total sample of national adults, one can say with 95% confidence that the maximum margin of sampling error is ±1 percentage point.
Interviews are conducted with respondents on land-line telephones (for respondents with a land-line telephone) and cellular phones (for respondents who are cell-phone only).
In addition to sampling error, question wording and practical difficulties in conducting surveys can introduce error or bias into the findings of public opinion polls.