Objectives: This study aimed to examine the joint effect of two core message elements -- authoritative source and argument strength -- in correction tweets to counter conspiratorial misinformation about the measles, mumps and rubella (MMR) vaccine. Design/Method: An online experiment with US residents (N = 404) was conducted in a 2 (authoritative correction sources: layperson vs US Centres for Disease Prevention and Control [CDC]) × 2 (correction argument strength: weak vs strong) design. Results: The results indicate that the correction employing strong arguments and a correction provided by the CDC heightened heuristic processing of the corrective information, which in turn increased the perceived credibility of the conspiratorial misinformation. The effect of the CDC correction on heuristic processing was heightened when it contained weak arguments. Notably, user-generated corrections with weak arguments reduced heuristic processing of the information and contributed to reducing the perceived credibility of the misinformation. Conclusion: Based on the findings, we argue that both communicator- and content-related cues jointly influence how audiences process corrective information. The current study discusses the potency of user-generated social media corrections to counter vaccine misinformation and provides practical implications for how user-generated social media correction can be utilised by health practitioners. Public health organisations should prioritise presenting corrective information in an easily understandable manner, using user-generated content that fosters a sense of connection and engagement with individuals.