Study reports the hospitalization risk for the delta versus alpha variants

In the absence of vaccination, any Delta outbreaks will impose a greater burden on healthcare.


A new study recently suggests that individuals contaminated with the SARS-CoV-2 delta variant have almost double the risk of hospitalization. Compared to an alpha variant, the risk is one and a half times greater.

This is the first study highlighting hospitalization risk for the COVID-19s Delta versus alpha variants based on confirmed cases. It validates a previous study that suggests people infected with Delta are significantly more likely to require hospitalization than those with Alpha, although most cases included in the analysis were unvaccinated.

This new study reviews healthcare data from 43,338 positive COVID-19 cases in England between 29 March and 23 May 2021. The data includes vaccination status, emergency care attendance, hospital admission, and other demographic characteristics. In all cases, samples of the virus taken from patients went through entire genome sequencing to affirm which variant had caused the infection.

There were 34,656 cases of the alpha variant (80%) and 8,682 cases of the delta variant (20%) during the study period. While the proportion of delta cases in the study period overall was 20%, it grew to account for around two-thirds of new COVID-19 cases in the week starting 17 May 2021 (65%, 3,973/6,090), indicating it had overtaken Alpha to become the dominant variant in England.

Around one in 50 patients were admitted to the hospital within 14 days of their first positive COVID-19 test (2.2% alpha cases, 764/34,656; 2.3% delta cases, 196/8,682). After accounting for factors that are known to affect susceptibility to severe illness from COVID-19, including age, ethnicity, and vaccination status, the researchers found the risk of being admitted to the hospital was more than doubled with the delta variant compared with the alpha variant (2.26-fold increase in risk).

In this study, only 1.8% (794/43,338) of COVID-19 cases (with either variant) had received both doses of the vaccine; 74% of cases (32,078/43,338) were unvaccinated, and 24% (10,466/43,338) were partially vaccinated. Hence, it was quite difficult for scientists to conclude how the hospitalization risk differs between vaccinated persons who later develop alpha and delta infections.

The results mainly suggest the risk of hospital admission for those who are unvaccinated or partially vaccinated.

Dr. Anne Presanis, one of the study’s lead authors and Senior Statistician at the MRC Biostatistics Unit, University of Cambridge, said: “Our analysis highlights that any Delta outbreaks will impose in the absence of vaccination greater burden on healthcare than an Alpha epidemic. Getting fully vaccinated is crucial for reducing an individual’s risk of symptomatic infection with Delta in the first place, and, importantly, of reducing a Delta patient’s risk of severe illness and hospital admission.”

There were several limitations to the study: some demographic groups may be more likely to seek hospital care, which could have biased the results, and there may have been changes in hospital admission policy during the period of the study, although adjustment for demographics and calendar time should have minimized such bias. Also, scientists did not have access to information about patients’ pre-existing health conditions, which affect the risk of severe illness from COVID-19.

They accounted for this indirectly using age, gender, ethnicity, and estimated level of socioeconomic deprivation.

Journal Reference:
  1. Hospital admission and emergency care attendance risk for SARS-CoV-2 Delta (B.1.617.2) compared with alpha (B.1.1.7) variants of concern: a cohort study, The Lancet Infectious Diseases (2021). DOI: 10.1016/S1473-3099(21)00475-8