The Monthly Outcome Survey (MOS) was designed to assess COVID-19 vaccine uptake as well as beliefs, intentions, and behaviors relevant to COVID-19 vaccination at a point in time. The survey fielded on a monthly basis from January 2021 to April 2023. When the MOS first launched, it focused on the primary series of COVID-19 vaccines; in later waves, it was expanded to assess parents’ intentions to get their children vaccinated or boosted and to track booster and updated vaccine uptake and readiness. The MOS fielded as part of an online omnibus survey, conducted with a cross-sectional sample of approximately 5,000 U.S. adults each month.