Safe and effective vaccines against SARS-CoV-2 are critical to ending the COVID-19 pandemic. Rare or serious outcomes associated with a vaccine, especially with the mRNA platform, may not be identified in phase 3 trials due to the limitation of follow-up duration and sample size. This study aims to explore the association between mRNA COVID-19 vaccines and the increased risk for serious health outcomes during days 1 to 21 after vaccination. The results show that the incidence of selected serious outcomes was not significantly higher 1 to 21 days post-vaccination compared with 22 to 42 days post-vaccination.