Key surveys overestimate COVID-19 vaccination rates in the USA

Estimates of COVID-19 vaccine uptake in the USA based on large surveys that are used to guide policy-making decisions tend to overestimate the number of vaccinated individuals, research published in Nature suggests. In the USA, the CDC (Centers for Disease Control and Prevention) compiles data on national vaccine uptake, but reporting can sometimes be delayed. Surveys that measure attitudes and behaviour towards COVID-19 vaccines can fill a gap when there is a lag in real-time data, and can inform government responses to the epidemic. However, some surveys diverge substantially in their findings. The authors of a new paper published in Nature today find that in May 2021, Delphi-Facebook's COVID-19 symptom tracker (250,000 responses per week) overestimated vaccine uptake by 17 percentage points, and a survey from the US Census Bureau (75,000 responses per wave) overestimated vaccine uptake by 14 percentage points compared to benchmark estimates from the CDC. These overestimation errors go orders of magnitude beyond the statistical uncertainty provided by the surveys. A survey by Axios-Ipsos also overestimated uptake, but by a smaller amount (4.2 percentage points in May 2021)-despite being the smallest survey (about 1,000 responses per week).