<b><i>Introduction:</i></b> As Switzerland faced the “second wave” of COVID-19 incidences, a discussion of a potential vaccine against the virus emerged. While some individuals accept vaccines, others challenge or refuse to be vaccinated, a phenomena called <i>vaccine hesitancy</i>. Here, trust plays a vital role in vaccination intention. Embitterment not only goes along with the sense of being treated unjust but also innates a distrust in others. Thus, embitterment may influence individuals’ vaccination intention against COVID-19. In the present study, we investigate how feelings of being socially excluded and the perceived negative impact of the pandemic are associated with embitterment and in turn, how embitterment is related to individuals’ vaccination intention and the tendency to hold COVID-19-related conspiracy beliefs (CCBs). This is in regard of the perceived communication style by the government. <b><i>Method:</i></b> A convenience sample of 281 individuals completed an online survey developed on Qualtrics. In this cross-sectional, nonexperimental designed study, indirect effects of a moderated serial mediation were analyzed using <i>Jamm</i> (Jamovi, Version 0.9; 2019). <b><i>Results:</i></b> Results indicated that embitterment went along with increased feelings of social exclusion (β = 0.45, <i>p</i> &#x3c; 0.001). Further, individuals high in embitterment generally indicated a higher vaccination intention against COVID-19 (β = 0.15, <i>p</i> &#x3c; 0.01). However, embittered individuals holding CCBs had a decreased vaccination intention against COVID-19 (β = −0.71, <i>p</i> &#x3c; 0.001). Thus, whether or not embittered individuals develop CCBs might be a crucial determinant for their vaccination intention. Noteworthy, the relationship between embitterment and the tendency to hold CCBs was reinforced by the notion of an unsatisfactory style of communication by the government. <b><i>Conclusion:</i></b> Taken together, results suggest that embitterment not only plays a relevant role in vaccination intention against COVID-19 but also for the susceptibility to engage in conspiracy beliefs.

There are indications for a raise in embitterment during the COVID-19 pandemic. As embitterment is related to felt social exclusion, pessimism, and a proneness toward COVID-19-related conspiracy beliefs, embitterment may be a key factor to consider in the current pandemic. However, perceived social support (PSS), which is associated with hope during the COVID-19 pandemic could serve as an important resilience factor for bitterness. We therefore investigated the effects of PSS on embitterment in aftermath of the second wave of the COVID-19 pandemic. 52 individuals participated in our follow-up online study in October 2020 (T1) and May 2021 (T2). Embitterment (measured with the Berne Embitterment Inventory) and PSS were repeatedly assessed at T1 and T2. A double moderation analysis was conducted, using the PROCESS macro to examine the effects of embitterment, PSS, and age on embitterment 6 months later. The results suggest that the prediction of embitterment at T2 by embitterment at T1 was altered by changes in PSS. Here, in particular participants with high embitterment at T1 and a gain in PSS had higher embitterment scores at T2, whereas participants with low embitterment were found to have lower embitterment scores. This interaction effect was further influenced by the age of the participants, where the effects were more pronounced among middle-aged participants than younger participants. As embittered individuals seem to grow in bitterness with a gain in PSS, support is found for the notion that embittered individuals seeking allies supporting their mindset.