This investigation highlights that the Delta variant of SARS-CoV-2 can spread quickly through a highly vaccinated population and can be transmitted to others regardless of vaccination status. Although vaccination remains a key mitigation strategy to decrease illness and death associated with COVID-19 (25), the Delta variant of SARS-CoV-2 is highly transmissible (26), and several studies have suggested lower vaccine effectiveness during Delta variant predominance compared with earlier months (5–7,27), probably driven by waning immunity from increased time since vaccination (28). In this outbreak, 99% of cluster-associated cases that had available sequencing were caused by the Delta variant, and 81% of cluster-associated cases were classified as vaccine breakthrough infections. The large number of breakthrough infections is probably representative of a highly vaccinated underlying population; as a greater proportion of the US population becomes fully vaccinated, vaccine breakthrough infections are likely to be more frequently observed (27,29). ...
In conclusion, major epidemiologic questions about breakthrough infections, such as the comparative infectiousness of fully vaccinated and non–fully vaccinated persons, duration of viral shedding, and duration of vaccine-derived immunity, remain. However, our findings underscore the need for persons who are fully vaccinated to take precautions to prevent transmission of SARS-CoV-2 to themselves and others, such as wearing a mask in public indoor settings or crowded outdoor settings, particularly during substantial or high transmission. Vaccination, although critical to reduce illness and death from COVID-19, should be complemented by layered mitigation strategies to address the COVID-19 pandemic (25,31).
