Unlike many other industrial societies, the partisan fights on the vaccination against COVID-19 in Taiwan centred on its brand choice rather than acceptance. Did the incumbent DPP supporters adhere to their party line of Medigen to vaccinate against COVID-19 during the pandemic? We argue that individual COVID-19 vaccine brand choices as gradually updated judgements during the pandemic were not solely determined by the party line, but jointly shaped by the strength of party affiliation and the level of government trust to facilitate decision-making in the highly uncertain information environment at the early stage of the pandemic. More specifically, when choosing COVID-19 vaccine brand, the incumbent DPP supporters, particularly weak ones, were more likely to adhere to their party line of Medigen as they trusted government more; however, this was only for the highly uncertain first shot, due to the absence of reliable information for making informed judgements about COVID vaccine brand choice at the early stage of the pandemic, not for the informationally rich booster shot. We report empirical findings consistent with our argument from statistical analyses of original data from a survey of 1642 Taiwanese adult respondents conducted in the fall of 2022.