The coronavirus vaccination programme is causing a "substantial" reduction in hospital admissions, a Scottish study has suggested.
Across the UK, more than 17.5 million people have received their first dose of the Pfizer-BioNTech or University of Oxford-AstraZeneca jab.
The vaccines were approved in the UK after research demonstrated the jabs significantly reduce the risk of developing severe disease with a coronavirus infection.
This appears to be bearing out, as scientists across Scotland have revealed the odds of hospitalisation were cut by up to 85% and 94% around four weeks after the first Pfizer-BioNTech and Oxford-AstraZeneca doses were administered, respectively.
England's chief medical officer Professor Chris Whitty has said the results "provide encouraging early data on the impact of vaccination on reducing hospitalisations".
While other experts agree, one has stressed now is not the time for complacency when it comes to adhering to social distancing restrictions.
The research comes after Nadhim Zahawi, minister for COVID vaccine deployment, told Sky News the UK's immunisation programme is "beginning to really bear fruit".
The preliminary results appear on the pre-print website SSRN and are yet to be published in a peer-reviewed journal.
"These results are very encouraging and have given us great reasons to be optimistic for the future," said lead author Professor Aziz Sheikh, from the University of Edinburgh.
"We now have national evidence, across an entire country, vaccination provides protection against COVID-19 [the disease caused by the coronavirus] hospitalisations.
"Roll-out of the first vaccine dose now needs to be accelerated globally to help overcome this terrible disease."
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In the first study to analyse the hospitalisation effects of the two vaccines across an entire country, the scientists used data from the EAVE II project to track Scotland's 5.4 million residents.
EAVE II uses patient data to monitor the pandemic and vaccine roll-out in real time.
The scientists specifically analysed the effects of a first vaccine dose between 8 December 2020 and 15 February 2021, when 1.14 million jabs had been administered and more than one in five (21%) Scottish people had received their first dose.
For each week of the over two-month period, the scientists tracked the GP records, hospital admissions, death registrations and laboratory tests results of those who had received their first vaccine versus the residents who had not been immunised.
The results suggest the Pfizer-BioNTech and Oxford-AstraZeneca jabs reduced the risk of a coronavirus-related hospitalisation by 85% and 94%, respectively, between 28 and 34 days post-vaccination.
Among those aged 80 or over, who are particularly vulnerable, the risk was cut by 81% when the results for both vaccines were combined.
The scientists believe their findings apply to other countries rolling out the same vaccines, however, the two jabs' effectiveness cannot be compared.
This comes after German officials said the Oxford-AstraZeneca vaccine should not be administered to people over 65 due to a lack of efficacy data in this age group.
"These results are important as we move from expectation to firm evidence of benefit from vaccines," said Dr Jim McMenamin, from Public Health Scotland (PHS).
"Across the Scottish population the results [have] shown a substantial effect on reducing the risk of admission to hospital from a single dose of vaccine.
"For anyone offered the vaccine I encourage them to get vaccinated.
"We are continuing our evaluation and look forward to describing the benefits we hope will follow the second doses of these vaccines."
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The results are particularly encouraging considering the study focused on the effects of just the first vaccine dose.
The jabs were approved based on a two-dose regimen, with an immunisation interval of around three weeks.
In an effort to get as many people vaccinated as possible, the UK's immunisation strategy has shifted to maximise first dose administrations, with up to a 12-week gap before the second jab.
"The discovery of very high protection before the second dose of the vaccines is very welcome news," said Professor Fiona Watt, from the Medical Research Council, which partially funded the study.
While the UK's vaccine roll-out has been hailed a success, an estimated 80% of the population must be immunised to create the much-lauded herd immunity.
"These data show real promise the vaccines we have given out can protect us from the severe effects of COVID-19," said Dr Josie Murray, from PHS.
"We must not be complacent though.
"We all still need to ensure we stop transmission of the virus and the best way we can all do this is to follow public health guidance – wash your hands often, keep 2m from others, and if you develop symptoms, isolate and take a test.
"We also all need to protect ourselves, our families and friends by taking the second dose of vaccine when it is offered."