People infected with omicron are less likely to require hospital treatment compared to patients who had delta, according to a large study published by U.K. health authorities on Friday.

The latest data from the U.K. Health Security Agency found the risk of hospitalization for people infected with omicron is about a third of that posed by the delta variant. The study analyzed more than 528,000 omicron cases and 573,000 delta cases from Nov. 22 through Dec. 26 in England.

However, Chief Medical Advisor Susan Hopkins cautioned that it is still too early to draw definitive conclusions about the severity of illness caused by omicron.

“The increased transmissibility of omicron and the rising cases in the over 60s population in England means it remains highly likely that there will be significant pressure on the NHS in coming weeks,” Hopkins said, referring to the U.K.’s National Health Service.

The World Health Organization on Wednesday also warned it is too early to conclude omicron is milder than past Covid variants. Dr. Abdi Mahamud, the WHO’s incident manager for Covid, said omicron has mostly infected younger people so far who generally develop less severe disease.

“We all want this disease to be milder, but the population it affected so far is the younger. How it behaves in the elderly population, the vulnerable — we don’t know yet,” Mahamud said during a news briefing in Geneva.

The new U.K. study also found Covid vaccines reduce the risk of hospitalization from omicron across the board, though a booster dose provides the highest level of protection. The latest data adds to a growing body of evidence demonstrating that although the vaccines have taken a hit from omicron, they still provide significant levels of protection compared to people who are unvaccinated.

One vaccine dose is 52% effective at preventing hospitalization from the omicron variant, while two doses were 72% effective, according to the study. After 25 weeks, however, the two doses weakened and were 52% effective at preventing hospitalization.

Booster doses significantly increase protection and are 88% effective at preventing hospitalization two weeks after receiving the shot, the study added.

“Protection against hospitalization from vaccines is good against the omicron variant,” the U.K. Health Security Agency concluded in the report.

However, the agency found the current vaccines are less effective at preventing symptomatic infection from omicron compared to the delta variant. The AstraZeneca vaccine, which is approved in the U.K. but not in the U.S., provides no protection against symptomatic infection from omicron 20 weeks after the second dose.

Pfizer and Moderna’s vaccines, the most widely administered shots in the U.S., are only about 10% effective at preventing symptomatic infection from omicron 20 weeks after the second dose. Booster doses, however, increase protection and are up to 75% effective at preventing infection two to four weeks after receiving a third shot. However, boosters weaken after about 10 weeks, providing 40 to 50% protection against symptomatic infection, the study said.

U.K. Health Secretary Sajid Javid said the unvaccinated are eight times more likely to end up in the hospital from Covid.