Delta doubles risk of severe COVID-19 among the unvaccinated

Pearl Toh
09 Sep 2021
Delta doubles risk of severe COVID-19 among the unvaccinated

The risk of hospitalization is more than doubled in patients infected with the Delta variant vs the Alpha variant, in particular among people who were unvaccinated, reveals a large cohort study — sounding the alarm that Delta is not only more transmissible, but also more virulent.

Among 43,338 COVID-19-positive patients (median age 31 years) in England with whole genome sequencing data available, Alpha cases made up 80 percent of the population and the remaining 20 percent were Delta cases during the study period between March to May 2021. However, by end May, Delta cases had grown to account for 65 percent of new COVID-19 infections, overtaking Alpha as the predominant circulating strain in England. [Lancet Infect Dis 2021;doi:10.1016/S1473-3099(21)00475-8]

Approximately one in 50 patients were hospitalized within 14 days of testing positive for COVID-19 — of which 2.3 percent were Delta cases and 2.2 percent were Alpha cases. After adjusting for potential confounders such as age, sex, ethnicity, and vaccination status, the risk of hospitalization was more than doubled from Delta vs Alpha infections (adjusted hazard ratio [aHR], 2.26, 95 percent confidence interval [CI], 1.32–3.89).

When looking at the combined outcome of emergency care visits or hospital admission, this risk was increased by 1.5 times in Delta cases compared with Alpha cases (aHR, 1.45, 95 percent CI, 1.08–1.95).

“These data confirm what we are seeing in clinical practice, namely that, in addition to the Delta variant being more infectious than the original or the Alpha variants, it is also causing more severe illness, in populations that previously would have had only mild infections,” commented Dr David Strain from the University of Exeter, UK, who was unaffiliated with the study.

“It highlights the need for a comprehensive vaccine program in younger adults and it clearly demonstrates the pre-conception that they do not get severe COVID is no longer true.”

When the analyses were stratified based on vaccination status, similar results were seen: the aHRs for emergency care attendance or hospital admission was 2.32 among unvaccinated patients and 1.94 among vaccinated patients.

Nonetheless, as most of the patients (74 percent) studied were unvaccinated, the authors said “the precision for the vaccinated subgroup was low” given they only account for 1.8 percent of the patients. Therefore, these findings applied primarily to those who were unvaccinated or partially vaccinated (24 percent).   

“Our analysis highlights that in the absence of vaccination, any Delta outbreaks will impose a 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,” said study author Dr Anne Presanis from the University of Cambridge, UK.

“It is still important that if you have COVID-19 symptoms, stay home and get a PCR test as soon as possible,” advised study co-author Dr Gavin Dabrera from the National Infection Service, Public Health England, UK.

“These findings should be considered for resource and policy planning in secondary care, particularly in areas where the Delta variant is increasing and is likely to become the dominant circulating SARS-CoV-2 variant,” urged Dabrera and co-authors.


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