The study, published in The Lancet Infectious Diseases journal on Friday, is an analysis of more than 40,000 coronavirus infections in England. It adds to evidence suggesting that Delta may cause more severe illness than other variants do.
Fewer than 2 percent of the infections occurred in fully vaccinated people, and there was not enough data to draw firm conclusions about hospitalization risks in that group specifically, the researchers said.
“The main takeaway is that if you have an unvaccinated or only partially vaccinated population, then an outbreak of Delta can lead to a higher burden on hospitals, on health care, than an Alpha outbreak would,” said Anne Presanis, a senior statistician at the University of Cambridge and one of the study’s lead authors.
The Delta variant, which was first detected in India, is roughly twice as infectious as the original virus and as much as 60 percent more transmissible than the Alpha variant, which was first identified in Britain.
In the new study, researchers analyzed the health data of people who tested positive for the virus in England from March 29 to May 23, as Delta was spreading through the country. Seventy-four percent of people were unvaccinated, 24.2 percent were partly vaccinated, and 1.8 percent were fully vaccinated.
Genetic sequencing confirmed that 80 percent of the study participants had been infected with the Alpha variant, while 20 percent had been infected with Delta.
But people infected with the Delta variant were younger, on average. When the researchers adjusted for age and other factors that are known to affect disease severity, they found that when Delta caused the infection, the overall risk of being hospitalized was more than two times as high.
When the researchers broke down the data by vaccination status, they confirmed that Delta doubled the risk of hospitalization for those who were unvaccinated or had received their first dose less than three weeks earlier.
“We already know that vaccination offers excellent protection against Delta, and as this variant accounts for over 98 percent of Covid-19 cases in the U.K., it is vital that those who have not received two doses of vaccine do so as soon as possible,” Dr. Gavin Dabrera, a lead author of the paper and an epidemiologist at Public Health England, said in a statement.