The SARS-CoV-2 Delta variant (B.1.617.2) was found to be associated with increased disease severity, increased viral load, and prolonged viral shedding compared with wild-type SARS-CoV-2, according to results of a retrospective cohort study published in Clinical Infectious Diseases. Findings also showed the COVID-19 vaccine to be protective against the Delta variant.

Investigators screened a cohort of patients with SARS-CoV-2 infection for variants of concern (VOC) who were admitted to the National Centre for Infectious Diseases (NCID) in Singapore. They included patients who were diagnosed with COVID-19 between December 20, 2020, when the Alpha (B.1.1.7) variant was first detected, and May 2021. The VOCs included in the analysis were Alpha, Beta (B.1.351), and Delta. The investigators performed whole genome sequencing among all patients with SARS-CoV-2 infection confirmed via polymerase chain reaction (PCR) testing with a cycle threshold (Ct) value of less than 30.

The investigators collected data from a second cohort of 846 consecutive patients admitted to the NCID for treatment of COVID-19 between January and April 2020. They compared outcomes of patients infected with a VOC in the national cohort with those with a VOC in the NCID cohort to better understand the relationship between COVID-19 variants, disease severity, and occurrence of viral shedding. Patients’ clinical data, laboratory results, and demographic information were included in the analysis. All patients received standard clinical management, with risk-stratification performed at the discretion of the treating physician.

The primary outcomes among patients in the national cohort and those in the NCID cohort were severe COVID-19 and development of pneumonia, respectively. Secondary outcomes were comparisons of viral PCR Ct values and duration of viral shedding among patients infected with a VOC. The investigators used a Ct value of greater than 30 as the cutoff for the end of an infectious period.

Among patients in the national cohort, 976 had COVID-19 with available DNA sequencing, of whom 829 (95%) were infected with a VOC: Alpha (20%), Beta (18%), and Delta (48%). The majority of patients infected with a VOC did not develop severe COVID-19.

After adjustment for age and sex, a multivariable logistic regression model showed that compared with patients who were not infected with a VOC, those infected with Delta had an increased risk for severe COVID-19, indicated by the need for supplemental oxygen, admission to an intensive care unit, or death (adjusted odds ratio [aOR], 4.90; 95% CI, 1.43-30.78).

Among patients in the NCID cohort, 157 were infected with a VOC: Alpha (7%), Beta (4%), and Delta (8%). Of note, the distribution of symptoms was similar among patients infected with a VOC vs those with wild-type virus.

 After adjustment for age, sex, comorbidities, and vaccination status,  Patients infected with Delta were found to have an increased risk for pneumonia compared with those with wild-type virus, though the risk was not statistically significant (aOR, 1.88; 95% CI, 0.95-3.76); also, Alpha and Beta variants were significantly less likely to be associated with an increased risk for pneumonia. In addition, patients infected with the Delta variant also had a significantly increased risk for developing severe COVID-19 infection (aOR, 3.02; 95% CI, 1.41-6.32).

Patients infected with Delta had a significantly increased duration of positive PCR tests with a Ct value of less than 30. The estimated median duration from symptom onset to first confirmed Ct value greater than30 was increased among patients with Delta compared with those with either wild-type virus (P =.01) or Alpha (P =.029), but similar to those infected with Beta. No significant difference in the estimated duration from symptom onset to first Ct value greater than 30 was noted among patients with Alpha vs those with Beta.

Of the 18 patients in the NCID cohort previously vaccinated against SARS-CoV-2 who developed breakthrough infections, 17 had received the Pfizer-BioNTech or Moderna vaccine and 1 had received the Oxford-AstraZeneca vaccine. All 18 patients with breakthrough infections developed mild disease only.

The study was limited by its small sample size, inconsistent SARS-CoV-2 testing and treatment due to its retrospective design, and the limited ability to adjust for initially unrecognized confounders between the 2 cohorts.

Because of the association between the Delta variant and prolonged viral shedding the investigators noted their support for “rapid implementation of vaccination programs,” and for infection control policies to adapt the recommended “duration of isolation of [patients with SARS-CoV-2 infection].”

Disclosure: One author declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of disclosures.


Ong SWX, Chiew CJ, Ang LW, et al. Clinical and virological features of SARS-CoV-2 variants of concern: a retrospective cohort study comparing B.1.1.7 (Alpha), B.1.315 (Beta), and B.1.617.2 (Delta). Clinical Infectious Diseases. Published online August 23, 2021. doi:10.1093/cid/ciab721