LONDON (REUTERS) - Two doses of Pfizer-BioNTech or AstraZeneca's Covid-19 vaccine are nearly as effective against the highly transmissible Delta coronavirus variant as they are against the previously dominant Alpha variant, a study published on Wednesday (July 21) showed.

Officials say vaccines are highly effective against the Delta variant, now the dominant variant worldwide, though the study reiterated that one shot of the vaccines is not enough for high protection.

The study, published in the New England Journal of Medicine, confirms headline findings given by Public Health England in May about the efficacy of Covid-19 vaccines made by Pfizer-BioNTech and Oxford-AstraZeneca, based on real-world data.

Wednesday's study found that two doses of Pfizer-BioNTech's shot was 88 per cent effective at preventing symptomatic disease from the Delta variant, compared with 93.7 per cent against the Alpha variant, broadly the same as previously reported.

Two shots of AstraZeneca vaccine were 67 per cent effective against the Delta variant, up from 60 per cent originally reported, and 74.5 per cent effective against the Alpha variant, compared with an original estimate of 66 per cent effectiveness.

"Only modest differences in vaccine effectiveness were noted with the Delta variant as compared with the Alpha variant after the receipt of two vaccine doses," Public Health England researchers wrote in the study.

Data from Israel has estimated lower effectiveness of Pfizer-BioNTech's shot against symptomatic disease, although protection against severe disease remains high.

The Public Health England had previously said that a first dose of either vaccine was around 33 per cent effective against symptomatic disease from the Delta variant.

The full study published on Wednesday found that one dose of Pfizer-BioNTech's shot was 36 per cent effective, and one dose of AstraZeneca's vaccine was around 30 per cent effective.

"Our finding of reduced effectiveness after the first dose would support efforts to maximise vaccine uptake with two doses among vulnerable groups in the context of circulation of the Delta variant," the authors of the study said.