The Delta variant may double a person's risk of being hospitalised with the coronavirus, but vaccines "still offer substantial protection".
The coronavirus emerged in Wuhan, China, at the end of 2019. It has since acquired mutations as it replicates, evolving into new variants.
At the end of 2020, the more transmissible Alpha variant emerged in Kent, quickly becoming the UK's dominant virus. This has since been replaced by the Delta variant, identified in India, which spreads more readily still.
The Delta variant is thought to be behind the UK's rising coronavirus cases, with 50,017 people testing positive on 13 June – a 49.3% increase on the week before.
The number of hospitalisations is also on the rise, with 1,008 coronavirus-positive people being admitted over the past seven days – a 15.2% increase. The extent to which the Delta variant is responsible is somewhat muddled, however.
Many are also concerned the Delta variant may bypass coronavirus vaccines, which were developed with the original Wuhan virus in mind.
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After analysing 5.4 million people, Scottish scientists found the Delta variant may more than double a person's risk of being hospitalised, compared to the Alpha virus.
Nevertheless, receiving two doses of either the Pfizer-BioNTech or University of Oxford-AstraZeneca vaccines provides strong protection against the Delta variant, but at a lower level than against the Alpha virus.
In a bid to get more people vaccinated, the lifting of restrictions in England – originally set for 21 June – is facing a delay of four weeks.
"Over a matter of weeks the Delta variant has become the dominant strain of [the coronavirus] in Scotland," said lead author Professor Aziz Sheikh, from the University of Edinburgh.
"It is unfortunately associated with increased risk of hospitalisation from COVID-19 [the disease caused by the coronavirus].
"Whilst possibly not as effective as against other variants, two doses of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines still offer substantial protection against the risk of infection and hospitalisation.
"It is therefore really important that when offered second doses people take these up both to protect themselves, and to reduce household and community transmission."
The UK has two other vaccines in its immunisation arsenal against the pandemic – Moderna and the single-dose Janssen. These were not included in the Scottish study due to the former only recently being rolled out and the latter being approved only on 28 May.
The Scottish study was carried out between 1 April and 6 June. During this time, more than 19,000 coronavirus community cases and 377 hospitalisations occurred where the responsible variant was confirmed.
Of these, more than 7,000 community cases and 134 hospitalisations were found to have the Delta variant. People with multiple medical conditions were more likely to require hospital care.
Both coronavirus vaccines were linked to a reduced risk of hospitalisation, but strong protection against the Delta variant did not emerge until at least 28 days after the first dose in the two-jab regimens.
Among the community cases, the Pfizer-BioNTech vaccine provided 79% protection against a Delta infection at least two weeks after the second dose.
This is compared to 92% protection against the Alpha variant, as published in the journal The Lancet.
The Oxford-AstraZeneca vaccine was 60% and 73% effective against the Delta and Alpha variants, respectively. It may take longer for this jab to bring about an immune response, according to the scientists.
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"These results provide early encouragement that two doses of either Pfizer-BioNTech or Oxford-AstraZeneca vaccines significantly reduce the risk of infection against both the Alpha or new Delta variants," said Dr Jim McMenamin, from Public Health Scotland.
"They also show the vaccines offer protection against the risk of hospitalisation with the new Delta variant.
"Though no vaccine can be 100% protective, they provide the best protection against COVID-19 and it remains important to get both doses when offered."
The two vaccines were not compared against each other due to "insufficient numbers of hospital admissions".
In addition, the scientists only analysed the link between variants, vaccines and hospitalisations – not deaths. It is unclear if an increase in hospital admissions will lead to a rise in fatalities, however, they could put the NHS under additional strain.
While the results are promising, the scientists have stressed further research is required.
"It is important to recognise these are preliminary results using rapidly accessible data," said co-author Professor Chris Robertson, from Strathclyde University.
"A fuller understanding will come when the results presented here are combined with similar analyses from other data sets in the UK."
In the meantime, the scientists will shortly present their findings to the World Health Organization.
"These results will inform decision making internationally," said co-author Professor Andrew Morris, from the University of Edinburgh.
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