Background: Infectiousness of respiratory viral infections is quantified as plaque forming units (PFU), requiring resource-intensive viral culture that is not routinely performed. We hypothesised that RNA viral load (VL) decline time (e-folding time) in people might serve as an alternative marker of infectiousness. Aim: This study''s objective was to evaluate the association of RNAVL decline time with RNA and PFU VL area under the curve (AUC) and transmission risk for SARS-CoV-2 and influenza A virus. Methods: In SARS-CoV-2 and influenza A virus community cohorts, viral RNA was quantified by reverse transcription quantitative PCR in serial upper respiratory tract (URT)-samples collected within households after an initial household-member tested positive for one virus. We evaluated correlations between RNAVL decline time and RNA and PFU-VL AUC. Associations between VL decline time and transmission risk in index-contact pairs were assessed. Results: In SARS-CoV-2 cases, we observed positive correlations between RNAVL decline time and RNA and PFUVL AUC with posterior probabilities 1 and 0.96 respectively. In influenza A cases a positive correlation between RNAVL decline time and RNAVL AUC was observed, with posterior probability of 0.87. Index case VL decline times one standard deviation above the cohort-mean showed a relative increase in secondary attack rates of 39% (95% credible interval (CrI):-6.9to95%) for SARS-CoV-2 and 25% (95% CrI:-11to71%) for influenza A virus. Conclusion: We identify VL decline time as a potential marker of infectiousness and transmission risk for SARS-CoV-2 and influenza A virus. Early ascertainment of VL kinetics as part of surveillance of new viruses or variants could inform public health decision making.
[1]Imperial Coll London, Natl Heart & Lung Inst, NIHR Hlth Protect Res Unit Resp Infect, London, England.
[2]Imperial Coll London, Dept Infect Dis, Sect Virol, London, England.
[3]Univ Oxford, NIHR Hlth Protect Res Unit Emerging & Zoonot Infec, Oxford, England.
[4]Imperial Coll London, Dept Infect Dis, Sect Struct & Synthet Biol, London, England.
[5]Imperial Coll London, UK Dementia Res Inst Ctr Care Res & Technol, London, England.
[6]Imperial Coll Translat & Innovat Hub, London Biofoundry, London, England.
[7]UK Hlth Secur Agcy, London, England.
[8]Imperial Coll London, Jameel Inst, MRC Ctr Global Infect Dis Anal, NIHR Hlth Protect Res Unit Modelling & Hlth Econ, London, England.
[9]Manchester Fdn Trust, Manchester Acad Hlth Sci Ctr, Dept Virol, Manchester Med Microbiol Partnership, Manchester, England.
[10]Univ Hong Kong, Li Ka Shing Fac Med, World Hlth Org Collaborating Ctr Infect Dis Epidem, Sch Publ Hlth, Hong Kong, Peoples R China.
[11]Lab Data Discovery Hlth D24H Ltd, Hong Kong Sci & Technol Pk, Hong Kong, Peoples R China.
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