Development and Validation of the Outcome Expectancy Scale for COVID-19 Vaccination in the Adult General Population

Promotion of COVID-19 vaccination requires understanding its determinants, an important one of which is outcome expectancy. However, reliable and valid measurement tools are absent. This study thus developed and validated an Outcome Expectancy Scale for COVID-19 Vaccination (OES-COVID-19). An inductive approach was used for scale development; content validity of the items was then assessed by an independent evaluation expert panel. Iteratively, 17 scale items were created and validated in a population-based telephone survey conducted among 500 adults of the general population in Hong Kong, China, from August to September 2021. Using half of the sample (n = 249), exploratory factor analyses identified four factors of the OES-COVID-19, including (a) positive contribution to society, (b) functional benefits, (c) protective effect, and (d) negative impacts. Confirmatory factor analysis of these factors conducted on the second subsample (n = 251) was satisfactory. The OES-COVID-19 showed good psychometric properties in terms of internal consistency, absence of floor/ceiling effects, and concurrent validity. The newly created and validated OES-COVID-19 is deemed suitable for application. It may advance future studies related to COVID-19 vaccination behavior and intention, and potentially allows for comparisons across studies. Further validation with modifications across countries, populations, and phase of the pandemic are warranted.