Different dynamics of mean SARS-CoV-2 RT-PCR Ct values between the first and second COVID-19 waves in the Madrid population.

SARS-CoV-2 RT-PCR cycle threshold values from 18,803 cases (2 March-4 October) in Madrid define three stages: (i) initial ten weeks with sustained reduction in viral load (Ct: 23.4-32.3), (ii) stability with low viral loads (Ct: 31.9-35.5) in the next nine weeks and (iii) sudden increase with progressive higher viral loads until reaching stability at high levels in the next twelve weeks, coinciding with an increased percentage of positive cases and reduced median age. These data indicate differential virological/epidemiological patterns between the first and second COVID-19 waves in Madrid.