Health communicators in the United States face substantial challenges in their efforts to increase parent uptake of the measles, mumps, and rubella (MMR) vaccine for their children. One major set of challenges involves low levels of trust in medical science behind vaccination safety and effectiveness, pharmaceutical companies who produce these vaccines, and government health agencies who promote vaccination. We conducted a two-wave randomized experiment (N = 1,000 at time 1, t1, N = 803 at time 2, t2) to test whether messages designed to convey the expertise, trustworthiness, or caring/goodwill of a governmental source of information (the Centers for Disease Control and Prevention) increased perceived source credibility, increased parent intentions to vaccinate their children, and/or reduced vaccine hesitancy. We found no support for any of the study's original, pre-registered hypotheses. However, post-hoc analyses reveal a variety of promising directions for future work on strategic messaging to increase source credibility in the context of vaccine hesitancy. A message designed to convey source expertise produced greater perceived caring/goodwill among parents overall. Furthermore, among parents who were vaccine hesitant at baseline, a message originally designed to convey source expertise produced greater perceived trustworthiness and reduced vaccine hesitancy among this group.