Although SARS-CoV-2 ‘RNAemia’ (viral RNA in blood) has been detected in patients with COVID-19, more often in severe disease, important questions remain: 1) Is the viral RNA in blood in cell-free virions? 2) Is the level of viremia prognostic? 3) Are viremia and neutralizing antibody titer related? We investigated these questions using multiple complementary approaches.
We developed an internally-controlled, ultrasensitive (1 copy/extraction) qRT-PCR assay for SARS-CoV-2 N gene RNA and applied this assay to plasma samples (0.5 – 1.0 ml) from COVID-19 outpatients and inpatients with variable severity of illness by WHO scale. For a subset of samples, we centrifuged plasma at 21,000xg for 2 hours, assayed viral RNA in both pelleted and supernatant fractions, and performed electron tomography on the pelleted fraction. SARS-CoV-2-specific antibody titers were determined using S1 and N EIAs and neutralizing antibody titers by infectious virus plaque reduction assay.
SARS-CoV-2 RNA was detected in plasma of 22/22 (100%) ICU patients, 9/18 (50%) non-ICU patients and 1/9 outpatients. Plasma viral RNA levels were significantly higher in ICU > non-ICU > outpatients (Fig 1A; p<0.0001 by Kruskal Wallis test), and among inpatients were strongly correlated with WHO score at admission (Spearman r=0.5338, p=0.0006), maximum WHO score during hospitalization (Fig 1B; Spearman r=0.6978, p<0.0001) and clinical outcomes of death, discharge to hospital-level care, discharge to home or discharge asymptomatic (p=0.0035 by Kruskal Wallis test). 80% of viral RNA was recovered in the pelleted fraction after centrifugation, and characteristic virions were observed in the pelleted fraction by electron tomography (Fig 1C). Neutralizing antibody titers showed no overall correlation with plasma viral RNA (Fig 1D) but revealed distinct subgroups with higher level viremia (>1000 copies/ml) and either low titer (<100) or higher titer (>100) neutralizing antibody.
SARS-CoV-2 viremia quantified by ultrasensitive RT-PCR was detected in 100% of ICU patients and 50% of non-ICU inpatients. The level of viremia correlated with disease severity and outcome, which may prove useful for clinical decision making. A subgroup of hospitalized patients with high-level viremia and low neutralizing antibody may be the best candidates for antibody therapy.