Delta variant doubles risk of COVID-19 hospitalization compared to alpha variant, study finds

People infected with the SARS-CoV-2 delta variant have approximately double the risk of hospitalization compared with those infected with the alpha variant, according to a new study of more than 40,000 cases from England between March 29, 2021 and May 23, 2021, published in the journal The Lancet Infectious Diseases.

The delta variant was first reported in India in December 2020 and early studies found it to be up to 50 percent more transmissible than the variant of COVID-19 that had previously gained dominance worldwide, known as the alpha variant, first identified in Kent in the United Kingdom.

A preliminary study from Scotland previously reported a doubling in risk of hospitalization with the delta variant compared with the alpha variant and it is suspected that delta is associated with more severe disease. The previous study used patients’ initial PCR test results and determined which variant they had by testing for a specific gene that is more common in the delta variant.

In the latest study, researchers analyzed healthcare data from 43,338 positive COVID-19 cases in England between March 29, 2021 and May 23, 2021, including information on vaccination status, emergency care attendance, hospital admission, and other demographic characteristics. In all cases included in the study, samples of the virus taken from patients underwent whole genome sequencing to confirm which variant had caused the infection.

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

Around one in 50 patients were admitted to hospital within 14 days of their first positive COVID-19 test. 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 hospital was more than doubled with the delta variant compared with the alpha variant.

Multiple studies have shown that full vaccination prevents both symptomatic infection and hospitalization, for both alpha and delta variants. Indeed, in this study, only 1.8 percent of COVID-19 cases, with either variant, had received both doses of the vaccine; 74 percent of cases were unvaccinated, and 24 percent were partially vaccinated. The authors note it is therefore not possible to draw statistically significant conclusions about how the hospitalization risk differs between vaccinated persons who later develop alpha and delta infections. The results from this study therefore primarily tell us about the risk of hospital admission for those who are unvaccinated or partially vaccinated.

The authors note 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. In addition, the authors did not have access to information about patients’ pre-existing health conditions, which are known to affect the risk of severe illness from COVID-19. They accounted for this indirectly using age, gender, ethnicity, and estimated level of socioeconomic deprivation.

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