ImportanceThe nasopharyngeal swab (NPS) is considered the ideal diagnostic specimen for Covid-19, while WMF is recently promoted due to collection simplicity and importance in disease transmission. There is limited knowledge on the relative viral load in these samples – NPS, whole mouth fluid (WMF) and respiratory droplets (RD; another important source in transmission), on how the loads vary with disease severity and on how much virus is shed.ObjectiveTo quantify and compare SARS-CoV2 copies in the NPS, WMF and RD samples, and correlate with disease severity.DesignCross sectional study.SettingTertiary care multi-speciality hospital with limited resources in a low-to-middle income country.ParticipantsEighty suspected COVID-19 patients were recruited from the COVID-19 out-patient clinic and hospital isolation wards.InterventionConcurrent NPS, WMF and RD samples were collected from all the recruited patients and tested for SARS-CoV2 copies by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR).Main outcomes and measuresThe main outcome was COVID-19 measured by SARS-CoV2 quantitative RT-PCR in NPS samples. COVID-19 disease severity was determined according to NIH criteria. Virus shedding was defined as the presence of SARS-CoV2 copies in the WMF and RD samples.ResultsSARS-CoV2 was detected in 55/80 (69%) of the NPS samples. Of these 55, WMF and RD samples were positive in 44 (80%) and 17 (31%), respectively. The concordance of WMF with NPS was 84% (p=0.02). SARS-CoV2 copy numbers were comparable in the NPS (median: 8.74×10^5) and WMF (median: 3.07×10^4), but lower in RD samples (median: 3.60×10^2). Patients with mild disease had higher copies in the NPS (median: 3.46×10^6), while patients with severe disease had higher copies in the WMF (median: 1.34×10^6) and RD samples (median: 4.29×10^4). The 25-75% interquartile range of NPS SARS-CoV2 copies was significantly higher in the WMF (p=0.0001) and RD (p=0.01) positive patients.Conclusion and relevanceSARS-CoV2 copies are highest in NPS samples. WMF is a reliable surrogate sample for diagnosis. High copy numbers in the NPS imply initial virological phase and higher risk of virus shedding via WMF and RD.Key pointsQuestionHow the numbers of SARS-CoV2 copies in nasopharyngeal swab (NPS) samples might reflectvirus shedding from the whole upper aerodigestive tract and indicatedisease severity?FindingsIn this cross-sectional study involving 80 suspected COVID-19 patients, the data indicate higher SARS-CoV2 copies in NPS samples of patients with mild disease,and in the whole mouth fluid (WMF) and respiratory droplet (RD) samples of patients with severe disease. Patients with higher SARS-CoV2 copies in the NPS shed the virus in the WMF and RD samples at statistically higher levels.MeaningHigh SARS-CoV2 copies in NPS samples imply initial virological phase withhigh levels of shedding through both WMF and RD.
