New UK data shows that the first dose of the Pfizer/BioNTech Covid-19 vaccine “provides high levels of protection against infection and symptomatic disease,” Public Health England (PHE) said in a news release on Monday.

PHE’s Siren Study, which was carried out on health care workers under the age of 65, found that one dose of the vaccine reduced the risk of infection by 72% after three weeks, while two vaccine doses reduced the risk of infection by 85%.

This high level of protection extended to the B.1.1.7 coronavirus variant first identified in the UK in December.

Health workers were tested for Covid-19 infection every two weeks using PCR tests and twice a week with lateral flow tests, explained Dr. Susan Hopkins, strategic response director at PHE, meaning “there was a lot of asymptomatic testing,” she said.

“Overall, we are seeing a really strong effect to reducing any infection: asymptomatic and symptomatic,” Hopkins said during a news conference held by the UK’s Science Media Centre on Monday.

PHE also analyzed routine testing data based on symptomatic disease in over 12,000 people, which showed that one dose of the Pfizer/BioNTech vaccine is 57% effective against symptomatic Covid-19 four weeks after the first dose in those older than 80. This increased to 88% one week after the second dose.

Early data has also shown that vaccinated people who are subsequently infected are far less likely to die or be hospitalized with the virus. People over the age of 80 who were infected post vaccination were 41% less likely to be hospitalized with the virus and 57% less likely to die of it.

PHE further predicted that protection against severe disease is likely to be over 75% in those who have received one dose of the Pfizer/BioNTech vaccine.

“The good sign is overall we are beginning to see a decline in hospitalizations and deaths in those vaccinated age groups [and] at least some is attributable to the vaccination program,” Dr. Mary Ramsay, head of immunization at PHE, said during the press conference.

She said that while the UK’s current lockdown played a part in the decline in hospitalizations, she added that “the faster speed in decline can be attributed in some respects to the vaccination program.”

In the PHE release, Ramsay cautioned that despite the encouraging signs “we don’t yet know how much these vaccines will reduce the risk of you passing Covid-19 on to others,” adding that vaccinated people should continue to follow stay-at-home orders.

The UK has placed its focus on vaccinating as many high-risk people as possible with the first dose.

UK Health Secretary Matt Hancock said it was “extremely encouraging” that data supported the UK government’s “decision to maximize the number of people vaccinated with a single dose.”