In this study, motivation for late (from 2021-W22, i.e., 24 July 2021) uptake of the first dose of the COVID-19 vaccine among adults in Poland and Lithuania is indirectly measured in order to avoid social-desirability bias or rationalisation in retrospect of prior decisions. Weekly vaccine uptake is modeled as if vaccine hesitant people were late adopters of a new product, with a fitted non-linear trend representing steadily decreasing interest. Before the analysed period, the vaccine uptake among Polish and Lithuanian adults was almost identical. Vaccination simply explainable by the trend was responsible for the vaccination of an additional 19.96% and 19.06% adults, respectively. The fear incurred by spikes in consecutive waves of infection motivated 3.20% and 3.89% more people, respectively, while the COVID-19 passport, introduced only in Lithuania, convinced an additional 13.98% of the overall population. The effect of the COVID passport was the biggest in the 18-24 age group, and the least visible among people aged 80 or more. In the latter group, other factors also had a limited impact, with merely 1.32% tempted by the one-time €100 payment offered to everybody aged 75 or more.
Keywords: COVID-19; interventions to increase vaccination coverage; public health; trust in vaccine; vaccination; vaccination coverage.