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SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness

Our take —

The highly transmissible Delta variant (B.1.617.2) of SARS-CoV-2, which was first identified from cases in India, has rapidly spread across the globe, replacing the Alpha variant (B.1.1.7) as the dominant lineage in Scotland by the end of May 2021. This study describes how the rise of the Delta variant in Scotland impacted case demographics, hospitalization rates, and vaccine effectiveness. Delta variant infection occurred more frequently among those who were young and affluent. Hospitalization risk was doubled for Delta variant cases when compared to Alpha variant cases, especially among persons with 5 or more comorbidities. Both the Pfizer- BioNTech and Oxford-AstraZeneca vaccines remained effective against breakthrough infection and hospitalization, though this effectiveness was reduced against the Delta variant as compared to the Alpha variant. This study demonstrates that the Delta variant of SARS-CoV-2 poses an increased risk to public health when compared to the Alpha variant, particularly for unvaccinated individuals.

Study design

Retrospective Cohort

Study population and setting

In late spring of 2021, the Delta variant (B.1.617.2) quickly spread across the globe, replacing the Alpha variant (B.1.1.7) as the dominant lineage in Scotland. This study used EAVE II surveillance data collected in Scotland between April 1 and June 6, 2021, to explore the impact of the Delta variant on risk of hospitalization and vaccine effectiveness. The Alpha variant genome contains a small deletion (residues 69-70) that prevents detection of the S gene with routine RT-qPCR diagnostic testing. In contrast, sequencing analysis demonstrated that Delta variant cases were 99% S gene positive and represented 97% of all S gene positive cases during the study period, allowing S-gene positivity to serve as a proxy for Delta variant detection. Cox regression was used to compare time to hospital admission by S gene status. Test-negative analysis was used to determine vaccine effectiveness against SARS-CoV-2 breakthrough infection as a function of S gene status, after adjusting for covariates.

Summary of Main Findings

During the study period, there were 19,543 observed cases of SARS-CoV-2 and 377 hospitalizations for COVID-19; 7,723 (39.5%) cases and 134 (35.5%) hospital admissions were attributed to infection with the Delta variant. Delta variant cases were associated with younger age (5-9 years) and socioeconomic affluence. COVID-19-related hospitalizations were also more frequent among Delta variant cases (HR: 1.85; 95% CI: 1.39-2.47) when compared to Alpha variant cases, after adjusting for comorbidities and demographic/temporal variables. Risk of hospitalization was greatest among individuals with 5 or more comorbidities. Both the Pfizer-BioNTech and Oxford-AstraZeneca vaccines were protective against breakthrough infection of both variants, but with reduced effectiveness against the Delta variant (Pfizer BioNTech: Alpha—92%, Delta—79%; Oxford AstraZeneca: Alpha—73%, Delta—60%).

Study Strengths

This study analyzed the impact of the SARS-CoV-2 Delta variant on hospitalization rates and vaccine effectiveness using a large, comprehensive dataset. Analyses were adjusted for several covariates that may skew the observed associations between variant status and outcomes.

Limitations

The Delta variant did not become dominant in Scotland until close to the end of the study period (on May 19, 2021), so the full extent of this variant’s impact on hospitalization and vaccine breakthrough infection may not have been adequately captured. The study does not identify any mechanisms that lead to increased hospitalization or vaccine breakthrough infection, and it is possible that the Delta variant influences disease presentation and testing pattern; if the Delta variant results in fewer symptomatic cases but more severe illness among those who are symptomatic, individuals infected with this variant who get tested for SARS-CoV-2 would appear to have worse outcomes in terms of hospitalization and vaccine breakthrough infection. In calculations of vaccine effectiveness, authors admit that temporal adjustments may not adequately account for trends in vaccine uptake and variant spread. No formal statistical analysis comparing the effectiveness of the two vaccines to each other was completed due to insufficient sample size for vaccine breakthrough cases.

Value added

This study describes the impact of the SARS-CoV-2 Delta variant (B.1.617.2) on case demographics, rates of hospitalization, and vaccine effectiveness in Scotland between April 1 and June 6, 2021 using nationwide case data.

This review was posted on: 11 July 2021