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Beginning in 2003, people age 5 to 49 who were healthy and not currently pregnant had the option of receiving protection against influenza via a nasal-spray vaccine as well as the traditional "flu shot" administered through a needle in the arm. Shifts in the proportion of people who reported receiving a "flu shot" before and after 2003 may reflect new reliance on the nasal-spray flu vaccine, rather than a real shift in the proportion of the population protected against the flu.

Beginning in 2004, the survey questionnaire explicitly distinguished between the flu shot and the nasal flu spray. Starting in that year, the survey form added the following text after the question for SHOTFLUYR: "Read if necessary: A flu shot is injected in the arm. Do not include an influenza vaccine sprayed in the nose."

Also beginning in 2004, the survey separately collected information about whether sample adults had received the nasal flu vaccine in the past 12 months (SPRAYFLUYR). Beginning in 2005, such information was also collected for sample children.

Beginning in 2005, according to the Field Representative's Manual, the computerized survey also included prompts to interviewers, leading them to ask a verification question about whether persons reported to have received a flu shot and the nasal flu spray in the past year had indeed received both types of flu vaccine.

For 1997 forward, interviewers always specified that a flu shot "is usually given in the fall and protects against influenza for the flu season"; in earlier years, this additional text was optional, to be read only "if necessary" for clarification.

In 1989, the respondent could answer for any person in the family, as the question was phrased, "During the past 12 months, that is, since (12-month date) a year ago, have any adults in the family received a flu shot?" For all later years, the respondent answered only for him or herself, when asked, "During the past 12 months, have you had a flu shot?"

Interviewers stated, "The next questions are about seasonal or regular flu vaccination. Please do not include H1N1 or swine flu vaccination." While the addition of this statement technically changes the wording of the question, including this text makes the 2009 results on flu vaccination more comparable to the results for earlier years than would otherwise be the case.