SARS-CoV-2 transmitters have more than three times higher viral loads than non-transmitters - Practical use of viral load for disease control

Abstract

Background: Quantitative results of SARS-CoV-2 testing reported as viral load copies/mL can provide valuable information, but are rarely used in practice. We analyze whether viral load in the upper respiratory tract is correlated with transmission and disease course and how this information can be used in practice.

Study design: Municipal Health Service (MHS) and clinical patients ≥18 years tested positive for SARS-CoV-2 with RT-PCR between June 1 and September 25, 2020 were included. Transmission was defined as an index having at least one contact tested positive. Test delay was defined as the time between symptom onset and SARS-CoV-2 testing.

Results: 683 patients were included (656 MHS and 27 clinical patients). The viral load was considerably lower among clinical patients compared to MHS patients: median log₁₀ copies/mL 2.51 (IQR -1.52 - 6.46) vs 4.92 (IQR -0.54 - 8.26), p < 0.0001. However, the test delay was higher for clinical patients (median 7 [IQR 2 - 19] vs 3 [IQR 0 - 26] days, p < 0.0001). SARS-CoV-2 transmitters showed much higher viral loads than non-transmitters (log₁₀ copies/mL 5.23 [IQR -0.52 - 8.26] vs 4.65 [IQR -0.72 - 8.00], p < 0.0001), but not for those with a test delay > 7 days. Higher viral loads were significantly correlated with older age and with more (severe) COVID-19 related symptoms.

Conclusion: Indexes that transmitted SARS-CoV-2 had more than three times higher viral loads than non-transmitters. Viral load information can be useful during source and contact tracing to prioritize indexes with highest risk of transmission, taking into account the test delay.

Keywords: COVID-19; SARS-CoV-2; Test delay; Transmission; Viral load.

Fig. 1

Test delay and associated viral load (log₁₀ copies/mL) for clinical and Municipal Health Service (MHS) patients.

Fig. 2

Correlation between viral load (log10 copies/mL) and patient characteristics and disease course. MHS: Municipal Health Service. Bold values denote statistical significance at the p < 0.05 level.

Fig. 3

Viral load (log₁₀ copies/mL) of indexes that transmitted SARS-CoV-2 and indexes that did not among the total study population (A) and the clinical versus Municipal Health Service (MHS) patients (B). Bold values denote statistical significance at the p < 0.05 level.

Fig. 4

Viral load (log₁₀ copies/mL) of indexes that transmitted SARS-CoV-2 and indexes that did not, presented by test delay categories (0–3 days, 4–7 days, > 7 days). Bold values denote statistical significance at the p < 0.05 level.

Fig. 5

Viral load (log₁₀ copies/mL) by secondary attack rate and contact category.