No difference in risk of hospitalisation between reported cases of the SARS-CoV-2 Delta variant and Alpha variant in Norway

Objectives

To estimate the risk of hospitalisation among reported cases of the Delta-variant of SARS-CoV-2 compared to the Alpha variant in Norway. We also estimated the risk of hospitalisation by vaccination status.

Methods

We conducted a cohort study on laboratory-confirmed cases of SARS-CoV-2 in Norway, diagnosed between 3 May and 15 August 2021. We calculated adjusted risk ratios (aRR) with 95% confidence intervals (CIs) using multivariable binomial regression, accounting for variant, vaccination status, demographic characteristics, week of sampling and underlying comorbidities.

Results

We included 7,977 cases of Delta and 12,078 cases of Alpha. Overall, 347 (1.7%) cases were hospitalised. The aRR of hospitalisation for Delta compared to Alpha was 0.97 (95%CI 0.76–1.23). Partially vaccinated cases had a 72% reduced risk of hospitalisation (95%CI 59%–82%), and fully vaccinated cases had a 76% reduced risk (95%CI 61%–85%), compared to unvaccinated cases.

Conclusions

We found no difference in the risk of hospitalisation for Delta cases compared to Alpha cases in Norway. Further research from a wide variety of settings is needed to better understand the association between the Delta variant and severe disease. Our results support the notion that partially and fully vaccinated persons are highly protected against hospitalisation with COVID-19.

Highlights

• The SARS-CoV-2 Delta variant has dominated in Norway since July 2021

• There was no difference in the risk of hospitalisation for Delta cases compared to Alpha

• Partially and fully vaccinated cases had >70% decreased risk of hospitalisation