Influence of the Delta Variant and Vaccination on the SARS-CoV-2 Viral Load

Abstract

Studies comparing SARS-CoV-2 nasopharyngeal (NP) viral load (VL) according to virus variant and host vaccination status have yielded inconsistent results. We conducted a single center prospective study between July and September 2021 at the drive-through testing center of the Toulouse University Hospital. We compared the NP VL of 3775 patients infected by the Delta (n = 3637) and Alpha (n = 138) variants, respectively. Patient's symptoms and vaccination status (2619 unvaccinated, 636 one dose and 520 two doses) were recorded. SARS-CoV-2 RNA testing and variant screening were assessed by using Thermo Fisher^® TaqPath™ COVID-19 and ID solutions^® ID™ SARS-CoV-2/VOC evolution Pentaplex assays. Delta SARS-CoV-2 infections were associated with higher VL than Alpha (coef = 0.68; p ≤ 0.01) independently of patient's vaccination status, symptoms, age and sex. This difference was higher for patients diagnosed late after symptom onset (coef = 0.88; p = 0.01) than for those diagnosed early (coef = 0.43; p = 0.03). Infections in vaccinated patients were associated with lower VL (coef = -0.18; p ≤ 0.01) independently of virus variant, symptom, age and sex. Our results suggest that Delta infections could lead to higher VL and for a longer period compared to Alpha infections. By effectively reducing the NP VL, vaccination could allow for limiting viral spread, even with the Delta variant.

Keywords: COVID-19; SARS-CoV-2; delta variant; vaccination; viral load.

Figure 1

SARS-CoV-2 RNA loads in nasopharyngeal specimens from infected individuals. Data are represented as violin plot with medians (thick midlines) plus interquartile range (IQR) (top and bottom dotted lines). The number of patients in each group and the p values for comparisons between groups (Mann–Whitney U-test) are shown. ns: not significant. (A) Viral loads in subjects with different variants (Alpha/Delta) and their immunization status. (B) Viral loads of unvaccinated COVID-19 patients infected with the Delta and Alpha variants, according to symptoms and sampling time after symptom onset. (C) Viral loads of unvaccinated and fully vaccinated COVID-19 patients infected with the Delta variant, according to symptoms and sampling time after symptom onset.