Secondary Attack Rates, Transmission, Incubation and Serial Interval Periods of first SARS-CoV-2 Omicron variant cases in a northern region of Spain.

SARS-CoV-2 Omicron variant (BA.1/B.1.1.529) quickly spread during December 2021 in Cantabria, a northern region of Spain. In total 622 cases were studied, and 1.420 close contacted were individually contact-traced and followed up. Secondary Attack Rates (SAR) and Transmission, Incubation and Serial Interval periods were estimated and compared with Delta cases. Greater transmission was found (Omicron SAR 39% vs 26%) with an increased tendency to asymptomatic or early infectiousness.