Abstract Background Individuals with intellectual and developmental disabilities (IDD) living in congregated settings have increased risk of COVID-19 infection and mortality. Little is known about variant B.1.1.519 with spike mutation T478K, dominant in Mexico. We describe a linked SARS-CoV-2 B.1.1.519 outbreak in three IDD facilities in the Netherlands. Methods Following notification of the index, subsequent cases were identified through serial PCR group testing. Positive specimens were submitted for whole-genome-sequencing. Clinical information was gathered through interviews with staff members of the three facilities. Results Attack rate (AR) in clients of the index facility was 92% (23/25), total AR in clients 45% (33/73) and in staff members 24% (8/34). 55% (18/33) of client cases were asymptomatic, versus 25% (2/8) of staff members. Five client cases (15%) were hospitalized, two died (6%). Sequencing yielded the same specific B.1.1.519 genotype in all three facilities. No significant difference in ...