Infections with the SARS-CoV-2 Delta variant exhibit fourfold increased viral loads in the upper airways compared to Alpha or non-variants of concern

Abstract

There has been a growing body of evidence that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta variant (B.1.617.2) shows enhanced transmissibility and increased viral loads compared to other variants. A recent study has even suggested that respiratory samples from people infected with the Delta variant can harbor up to 1000 times higher viral loads compared to samples with variants that are more closely related to the original Wuhan strain, although the sample size of this study (n = 125) was very limited. Here, we have compared the viral load in 16,185 samples that were obtained in periods during which non-VOC, the Alpha (B.1.1.7) or Delta variant (B.1.617.2) were dominant as evidenced by genomic surveillance. We found that the Delta variant contained about fourfold higher viral loads across all age groups compared to the non-VOC or Alpha variants, which is significantly lower than reported earlier. Interestingly, the increased viral load for the Delta variant seemed to be age-dependent, regardless of sex, as the viral load was about 14-fold higher for Delta compared to the non-VOC or Alpha variant in age group 0-20 years and fourfold higher in age group 21-40 years, while there was no difference in viral load between variants in age groups 41-60 and 61+ years, most likely as a consequence of a higher degree of vaccination in the older age groups.

Figure 1

Distribution of circulating variants in the South Limburg region in time. The period during which a particular variant was dominant (> 50% of sequenced samples) is indicated below. Lines represent the relative proportion of each variant, whereas bars represent the total number of samples that were tested positive (grey) or were sequenced (yellow) in function of time. At least 7% of positive samples were sequenced per week.

Figure 2

CT values (A) and viral loads (B) per time period dominated by non-VOC (Dec 2020–Feb 2020), Alpha (Mar 2020–Jun 2020) and Delta (Jul 2020) SARS-CoV-2 variants. CT values (C) and viral loads (D) per WGS confirmed SARS-CoV-2 variants. Ns not significant, **p < 0.01, ***P < 0.001, ****p < 0.0001. All grouped analyses had a power > 0.99.

Figure 3

CT values per time period dominated by non-VOC (Dec 2020–Feb 2020), Alpha (Mar 2020–Jun 2020) and Delta (Jul 2020) SARS-CoV-2 variants for age groups 0–20 (A), 21–40 (B), 41–60 (C) and 60+ years (D). Ns not significant, **p < 0.01, ***P < 0.001, ****p < 0.0001.