From the Background: "As of the end of March 2021, >30 million coronavirus disease 2019 (COVID-19) cases and >546,000 COVID-related deaths had been reported in the United States. Safe and effective COVID-19 vaccines are an important tool in controlling the pandemic. The U.S. Food and Drug Administration (FDA) has issued Emergency Use Authorization (EUA) for three COVID-19 vaccines. In large, randomized controlled trials, each vaccine was safe and efficacious for preventing symptomatic, laboratory-confirmed COVID-19 disease. There were no statistically significant differences in efficacy by age, gender, race, or comorbidities. Despite the high level of efficacy, a small number of breakthrough cases occurred. The phase 3 studies have not yet identified risk factors for vaccine breakthrough cases; however, approximately 44,000 people received the FDA-authorized vaccines during the clinical trials and follow-up is ongoing. In addition, several other COVID-19 vaccines are in late-stage development. [...] This protocol can be used to guide public health investigations of COVID-19 vaccine breakthrough cases."
Centers for Disease Control and Prevention: https://www.cdc.gov/