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

Cameron Buckley; Claire Yt Wang; Mark D Chatfield; Cheryl Bletchly; Patrick Harris; David Whiley

doi:10.1016/j.diagmicrobio.2021.115598

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

Diagn Microbiol Infect Dis. 2022 Mar;102(3):115598. doi: 10.1016/j.diagmicrobio.2021.115598. Epub 2021 Nov 19.

Authors

Cameron Buckley¹, Claire Yt Wang², Mark D Chatfield³, Cheryl Bletchly⁴, Patrick Harris¹, David Whiley⁵

Affiliations

¹ Faculty of Medicine, The University of Queensland Centre for Clinical Research, The University of Queensland, Brisbane, Queensland, Australia; Pathology Queensland Central Laboratory, Brisbane, Queensland, Australia.
² Pathology Queensland Central Laboratory, Brisbane, Queensland, Australia; Centre for Children's Health Research, Children's Health Queensland, Brisbane, Queensland, Australia.
³ Faculty of Medicine, The University of Queensland Centre for Clinical Research, The University of Queensland, Brisbane, Queensland, Australia.
⁴ Pathology Queensland Central Laboratory, Brisbane, Queensland, Australia.
⁵ Faculty of Medicine, The University of Queensland Centre for Clinical Research, The University of Queensland, Brisbane, Queensland, Australia; Pathology Queensland Central Laboratory, Brisbane, Queensland, Australia. Electronic address: d.whiley@uq.edu.au.

Abstract

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.

Keywords: SARS-CoV-2; diagnosis; phase; viral load.

MeSH terms

COVID-19 Testing
COVID-19*
Humans
Pandemics
SARS-CoV-2*
Viral Load