Pinpointing the Infectiousness of SARS-CoV-2 and Its Alpha (B.1.1.7) Variant

Determining the infectiousness of individuals with COVID-19 requires considering degree of symptoms, time point within disease course, and whether the infection involves wild-type SARS-CoV-2 or a variant. To clarify these criteria, investigators in Germany assessed viral loads and cell culture infectivity in samples from 25,381 COVID-19 cases (including, among others, 9519 who were hospitalized; 6110 who were presymptomatic, asymptomatic, or mildly symptomatic [PAMS]; and 1533 who were infected with the Alpha [B.1.1.7] variant of concern).

Among all cases, mean viral load in the first positive sample was 6.39 (log₁₀ RNA copies per swab). Among those aged 20–65, viral load was higher in PAMS than in hospitalized individuals; in general, children and adolescents had lower viral loads than adults. Regardless of age, the probability of positive cultures from first positive samples was 0.35 (higher for PAMS than hospitalized individuals). In all, 2228 individuals had viral loads ≥9.0 and culture positivity probability of 0.92; 36% of these individuals were PAMS. Among cases infected with the Alpha variant, mean first-positive viral load was 7.38 (1.05 log₁₀ higher than non-Alpha cases), and the probability of culture positivity was 0.50 (compared with 0.31 for non-Alpha cases). For 4344 individuals with at least 3 positive RT-PCR samples, estimated time from onset of shedding to peak viral load was 4.3 days (mean peak viral load, 8.0). The mean probability of culture positivity progressed from 0 to a peak of 0.74 over 2 days, declining to 0.52 at day 5 and 0.29 at day 10 thereafter.