Two doses of Pfizer 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 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's shot was 88% effective at preventing symptomatic disease from the Delta variant, compared to 93.7% against the Alpha variant, broadly the same as previously reported.

Two shots of AstraZeneca vaccine were 67% effective against the Delta variant, up from 60% originally reported, and 74.5% effective against the Alpha variant, compared to an original estimate of 66% 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's shot against symptomatic disease, although protection against severe disease remains high.

PHE had previously said that a first dose of either vaccine was around 33% effective against symptomatic disease from the Delta variant.

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