Contact tracing data of SARS-CoV-2 Omicron variant cases during December 2021 in Cantabria, Spain, showed increased transmission (secondary attack rate 39%) compared with Delta cases (secondary attack rate 26%), uninfluenced by vaccination status. Incubation and serial interval periods were also reduced. Half of Omicron transmissions happened before symptom onset in the index case-patient.
Keywords: COVID-19; Omicron variant; SARS-CoV-2; Spain; coronavirus disease; incubation period; respiratory infections; secondary attack rate; severe acute respiratory syndrome coronavirus 2; surveillance; viruses; zoonoses.