The impact of COVID-19 epidemic phase and changes in mean viral loads: implications for SARS-CoV-2 testing strategies

The sensitivity of SARS-CoV-2 diagnostic tests is inherently linked to viral load. We explored whether average viral loads changed at a population level in Queensland, Australia during the early phase of the pandemic. RT-PCR threshold cycle (C(T)) values, a crude marker for viral load, were compared for samples collected in February/March-2020 to those collected in April/May-2020, noting that the major public health interventions began in late-March 2020. Positive detections peaked mid-March, which coincided with the highest detection numbers and lowest C(T) values. However, this changed from April where the later C(T) samples (C(T) > 30) predominated. Overall, in February/March 29% (267/922) of samples had C(T) values >30 cycles compared to 88% (559/636) in April/May. Our study shows that SARS-CoV-2 viral loads in patients may vary at a population level over time. This needs considering when assessing suitability of diagnostic methods, particularly when methods in question are known to have reduced sensitivity.