A large UK study into rare blood clots linked with AstraZeneca's COVID-19 vaccine has found between just one and three cases per million, and only after the first dose, Reuters news agency reported on Wednesday.

Researchers looked for any link between COVID-19 vaccines and rare blood clots in the brain, arteries or veins, sometimes accompanied by low platelets.

Reports of blood clots led many countries last year to pause use of the AstraZeneca (LON:AZN) COVID-19 vaccine, which was developed with Oxford University.

A study published in the PLOS Medicine journal on Tuesday looked at health records of 46 million adults in England between December 2020 and March 2021 to assess the risk of clots in the month after vaccination with either the Pfizer (NYSE:PFE)-BioNTech (Nasdaq:BNTX) vaccine or AstraZeneca-Oxford shot, as compared with the unvaccinated.

The study, carried out by William Whiteley of the University of Edinburgh and Britain's BHF Data Science Centre, found no risk of major arterial and venous thrombotic events in those aged 70 or over with either of the vaccines.

According to the study, while the risk of intracranial venous thrombosis (ICVT) following the AstraZeneca-Oxford vaccine was nearly double in those under 70, it was equal to between just one and three cases per million.

The study's authors said that the risks of ICVT and hospitalisation with thrombocytopenia "are likely to be outweighed by the vaccines' effect in reducing COVID-19 mortality and morbidity."

A second peer-reviewed UK study also released on Tuesday found the risk of cerebral venous sinus thrombosis (CVST) in the four weeks after receiving the AstraZeneca-Oxford shot was roughly twice as high as before vaccination, but it still implied only one in four million people could have the side-effect.

This study assessed data for more than 11 million people in England, Scotland and Wales, spanning nearly seven months from December 2020 to June 2021.