We analyze interviews with participants in a COVID-19 vaccine trial to show how Americans navigate conflicting discourses of individual rights and collective responsibility by using individual health behavior to care for others. We argue that interviewees drew on ideologies of "collective biology" - understanding themselves as parts of bio-socially interrelated groups affected by any member's behavior - to hope their participation would aid collectives cohering around kinship, sex, age, race and ethnicity. Benefits (protecting family, representing one's group in vaccine development and modeling vaccine acceptance) existed alongside drawbacks (strife, reifying groups), to illustrate the ambivalence of caregiving amid inequality.
Keywords: COVID-19; care; individualism; medical research; responsibilization; vaccine.