Table 1.

Survey measures.

Concept/variable	Type of Index & # of items	Questions	Response categories
Independent
Twitter Use	1 Question	1. Do you have a Twitter account?	No or Yes (0 or 1)
Twitter - Health Information	Collapsed Response of 3 Questions	1. Do you use Twitter to follow or like health agencies or health-related organizations? 2. Do you use Twitter to follow flu or other illness outbreaks? 3. Do you use Twitter to get information about vaccines?	No or Yes (0 or 1)
Facebook Use	1 Question	Do you have a Facebook account?	No or Yes (0 or 1)
Facebook - Health Information	Collapsed Response of 3 Questions	1. Do you use Facebook to follow or like health agencies or health-related organizations? 2. Do you use Facebook to follow flu or other illness outbreaks? 3. Do you use Facebook to get information about vaccines?	No or Yes (0 or 1)
Social Media Importance	1 Question	1. When it comes to deciding whether or not to get vaccinated for flu, how important is the information you get on Facebook and/or Twitter?	Not at all important or a little bit important (0) - Extremely important or fairly important (1)
Covariate
Influenza knowledge	Sum of Correct Responses (0–7)	1. The flu vaccine helps stimulate a natural immune response? 2. A flu vaccine will protect you from the flu for many years? 3. The flu vaccine does not include all the types of flu circulating in the US this year? 4. Flu vaccines must be tested and approved every year? 5. Flu vaccines change every year because the type of flu virus change all the time? 6. Even if the flu vaccine does not contain all types of virus going around it can still help reduce the seriousness and length of time I am sick if I get the flu? 7. The flu vaccine this year is less effective than most years?	True or False
Outcome: Vaccine behavior	1 Question	1. Did you get a flu vaccine this season?	No or Yes (0 or 1)