Viral loads and profile of the patients infected with SARS-CoV-2 Delta, Alpha or R.1 variants in Tokyo

The rapid spread of the Delta variant of SARS-CoV-2 became a serious concern worldwide in summer 2021. We examined the copy number and variant types of all SARS-CoV-2-positive patients who visited our hospital from February to August 2021 using PCR tests. Whole genome sequencing was performed for some samples. The R.1 variant (B.1.1.316) was responsible for most infections in March, replacing the previous variant (B.1.1.214); the Alpha (B.1.1.7) variant caused most infections in April and May; and the Delta variant (B.1.617.2) was the most prevalent in July and August. There was no significant difference in copy numbers among the previous variant cases (n=29, median 3.0x104 copies/L), R.1 variant cases (n=28, 2.1x105 copies/L), Alpha variant cases (n=125, 4.1x105 copies/L), and Delta variant cases (n=106, 2.4x105 copies/L). Patients with Delta variant infection were significantly younger than those infected with R.1 and the previous variants, possibly because many elderly individuals in Tokyo were vaccinated between May and August. There was no significant difference in mortality among the four groups. Our results suggest that the increased infectivity of Delta variant may be caused by factors other than the higher viral loads. Clarifying these factors is important to control the spread of Delta variant infection.