Delta variant and hospitalization rates: Study investigates

  • Since its emergence, the SARS-CoV-2 virus has mutated to produce a number of variants, including the Delta variant.
  • A new study suggests that unvaccinated individuals with a Delta variant infection are at a higher risk of hospitalization than those with an Alpha variant infection.
  • However, the study does have certain limitations.

Over the course of the COVID-19 pandemic, the SARS-CoV-2 virus has mutated. Much of the latest research and concern has revolved around the Delta variant.

Scientists are still investigating how easily it transmits from person to person and whether the severity of symptoms it produces differs from the severity of symptoms other variants cause.

A recent study, which appears in the journal The Lancet Infectious Diseases, suggests that people who contract the Delta variant are more likely to be hospitalized.

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The Delta variant

To help track SARS-CoV-2 variants, the World Health Organization (WHO) has created the Variant of Concern (VOC) classification. To fall into a VOC category, a SARS-CoV-2 variant must meet at least one of the following:

  • It has an increased transmissibility or creates a significant change to COVID-19 epidemiology.
  • It has an increased ability to cause damage to people who contract the virus, or has changed how the disease presents in a clinical setting.
  • It reduces the effectiveness of public health measures, social measures, or treatments.

Two variants that fall into this category are the Alpha variant and the Delta variant. Authors of the study note that the data available on the Delta variant are limited. There are few studies comparing the severity of COVID-19 symptoms caused by the Delta variant with the severity of symptoms that the Alpha variant brings about.

According to the Centers for Disease Control and Prevention (CDC), the Delta variant is more contagious than the Alpha variant.

The CDC also cites two studies that suggest the Delta variant may cause more severe illness in unvaccinated individuals.

Increased risk of hospitalization

In England, where the current study took place, scientists identified the Alpha variant in November 2020.

By February 2021, the Alpha variant accounted for about 95% of COVID-19 cases. Until recently, the Alpha variant has been the dominant variant of SARS-CoV-2 in Europe and North America.

The Delta variant was first detected in England in March 2021, and by late May, it was designated as a VOC. By the end of May, it was responsible for over 50% of COVID-19 cases.

Authors of the current study sought to determine whether those who contracted the Delta variant had a greater risk of hospitalization, compared with those who contracted the Alpha variant. They also wanted to determine whether vaccination modified the associated risk.

The cohort study included individuals in England with COVID-19 between March 29 and May 23, 2021, who had contracted either the Alpha or Delta variant of SARS-CoV-2.

This included 8,682 individuals with the Delta variant and 34,656 individuals with the Alpha variant. The researchers confirmed the variant using whole-genome sequencing.

Additionally, the study authors examined the rates of hospital admission and emergency room (ER) care among those with the Alpha and Delta variants.

Individuals were considered to be receiving emergency or hospital care if they came to the ER or were admitted to the hospital 1–14 days after their initial test for SARS-CoV-2, and the reason for care was not related to injury.

Overall, the hospitalization rates were higher among people with the Delta variant. The analysis showed that the risk of hospitalization was about twice as high for the Delta variant cases, compared with the Alpha variant cases.

When the researchers expanded this to include risk of hospitalization or ER care, the risk was about 1.45 times higher for those with a Delta variant infection.

Although the results appear concerning, when asked to comment on the study, epidemiology specialist with Johns Hopkins University Dr. David W. Dowdy was optimistic.

He noted that overall hospitalization rates in the United Kingdom have gone down, which he believes to be due to vaccination efforts. He explained to Medical News Today:

“While this study seems to have been performed well, it’s also important to recognize that people at higher risk of hospitalization are now also more likely to be vaccinated. As a result, even though the Delta variant now accounts for most cases in the U.K. and many other countries, the fraction of people with COVID-19 who are being hospitalized has declined substantially in the U.K. — at the same time as the most recent Delta surge.”

He continued: “Thus, any effect of the Delta variant causing more serious illness is being more than counteracted by the effects of vaccination, on the population level. The average person diagnosed with COVID-19 in the U.K. is about 40% as likely to be hospitalized now, versus during the surge in the winter of 2020–21. In the U.K., the severity of COVID-19 illness as a whole is going down, not up.”

Study limitations and research moving forward

Most of the study participants were either unvaccinated or only partially vaccinated. The researchers note that the risk rates of hospitalization or emergency care attendance were similar in the vaccinated and unvaccinated subgroups.

However, the precision of the risk assessment for vaccinated individuals was low. This indicates that more research may be necessary to determine whether there is a greater risk of hospitalization for vaccinated individuals with a Delta variant infection, compared with the Alpha variant.

The study did have other limitations. For instance, some groups may have been underrepresented because of the way in which data were collected. The study authors acknowledge that there may have been delays in collection of hospitalization and ER attendance, thus increasing the risk of inaccuracy.

There may also have been reasons for hospitalization or emergency care not related to COVID-19 being included in analysis, but this would have impacted both the Alpha and Delta groups equally.

Also, the researchers did not have data on the comorbidities of the study participants, and these tend to contribute to hospitalization risk. However, they note that they “indirectly accounted for comorbidities using related covariates, including age, sex, ethnicity, and deprivation.”

They further warn that there should be caution in generalizing their results due to the conditions surrounding and the success of the whole-genome-sequencing selection. Moreover, there was the possibility of overrepresentation, which could have led to an overestimation of risks of hospitalization or emergency care use.

Authors of the study note that future research should also investigate a range of outcomes for people with COVID-19, such as length of hospitalization, admittance to intensive care, and mortality rates.

Infectious disease expert Dr. Arturo Casadevall concurred with this and explained to MNT:

“This is an important study from the U.K., establishing that the […] Delta variant is more likely to lead to hospitalization. It will be important to know whether mortality with Delta variant is also higher and whether existing therapies developed against earlier versions of SARS-CoV-2 are as effective against this variant.”

Overall, despite limitations, the study adds to the growing amount of data about the Delta variant that experts now have at their disposal.

Authors of the study note, “To our knowledge, this study is the largest assessment of hospitalization risk for the Delta variant using cases confirmed by whole-genome sequencing, providing important foundational evidence of increased risk compared with the Alpha variant.”

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