The study by Oxford University researchers, published in the journal Cell, investigated the ability of antibodies in the blood from people, who were vaccinated with the two-shot regimens, to neutralise the highly contagious Delta and Kappa variants, a statement said.

"There is no evidence of widespread escape suggesting that the current generation of vaccines will provide protection against the B.1.617 lineage," the paper says, referring to the Delta and Kappa variants by a commonly used code.

However, the concentration of neutralising antibodies in the blood was somewhat reduced, which may lead to some breakthrough infections, they cautioned.

Last week, an analysis by the Public Health England showed that vaccines made by Pfizer and AstraZeneca offer high protection of more than 90 per cent against hospitalisation from the Delta variant.

"We are encouraged to see the non-clinical results published from Oxford and these data, alongside the recent early real-world analysis from Public Health England, provide us with a positive indication that our vaccine can have significant impact against the Delta variant," AstraZeneca executive Mene Pangalos said in a separate statement.

The Delta variant is becoming the globally dominant version of the disease, the World Health Organisation's chief scientist said on Friday.

The Oxford researchers also analysed reinfection patterns in people who had previously had COVID-19. The risk of reinfection with the Delta variant appeared particularly high in individuals previously infected by the Beta and Gamma lineages that emerged in South Africa and Brazil, respectively.

By contrast, previous infection with the Alpha, or B117, variant first detected in Britain, conferred "reasonable" cross-protection against all variants of concern, lending itself as a template that next-generation vaccines could be molded on.

"B117 might be a candidate for new variant vaccines to provide the broadest protection," the researchers said.

Australian Associated Press