But real-world data from four countries where the variant is either dominant or on the rise indicates that vaccinations can protect against the most severe forms of COVID-19 on a country-wide scale.
Russia, Indonesia, Britain, and Israel have been dealing with a surge in cases, which they attribute to the Delta variant.
But while Russia and Indonesia have both seen a sharp uptick in the number of COVID-19 related daily deaths, the UK and Israel have not.
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Israel has only seen a small rise in daily new recorded infections, reporting a weekly average of 99 new cases a day on Thursday, compared to 16 in the previous week - which could explain why deaths have not risen significantly.
But the UK has been reporting more daily new infections, recording on Thursday a weekly average of 12,009 new cases a day. Russia and Indonesia, meanwhile, have reported weekly averages of 17,546 and 14,817 daily new cases, according to Our World in Data.
The difference between the countries' death rates could be down to vaccination.
Despite vaccines being available since December, Russia's vaccination campaign has stalled. As of Wednesday, just 11% of its population has been fully vaccinated, and 14% have received one dose of COVID-19 vaccine, the Associated Press reported.
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Indonesia, meanwhile, has been struggling to obtain vaccines. As of Wednesday, just 9% of people in Indonesia have received one dose of vaccine, and 4% have been fully vaccinated, Our World in Data reported.
By comparison, the UK and Israel have among the highest rates of vaccination in the world, with 46% fully vaccinated, and 64% having received one dose in the UK; and 59% fully vaccinated, and 63% having received one dose in Israel, according to Our World in Data.
Vaccines seem protective against severe COVID-19
Data from the UK suggests the Delta variant is more likely to break through the protection given by a single shot of Pfizer or AstraZeneca vaccine, conferring only 33% protection against symptomatic disease, as Insider's Aria Bendix reported.
But when it comes to protection against hospitalization, even a single dose of both of those vaccines is still highly protective: the AstraZeneca shot offers 71% efficacy against hospitalization, while the Pfizer shot offers 94% protection, data from the UK shows.
Data on the efficacy of other COVID-19 vaccines has not been made available.
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The makers of the Russian vaccine Sputnik V have said that it is "more efficient against the Delta variant," though data to support this claim has not been released.
Indonesia recently reported that hundreds of healthcare workers vaccinated with the Chinese vaccine Sinovac caught the coronavirus. The majority of these did not have symptoms, but dozens needed to be hospitalized, Insider's Sarah Al-Arshani reported.
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