Newly-published research from France's Pasteur Institute suggests that people are mostly protected from the Delta variant of coronavirus after two doses of a COVID-19 vaccine.
The study, published in Nature journal on Thursday, tested the effects of exposure to the hyper-infectious strain first identified in India on 59 vaccinated people in Orléans, France.
Sixteen of the participants had received at least one dose of Pfizer and 43 individuals had been given one shot or more of AstraZeneca.
The researchers found that for both Pfizer and AstraZeneca recipients, the levels of neutralising antibodies produced against the Delta variant were "almost undetectable".
But after receiving the second dose, aro 95 per cent of people were able to neutralise the Delta variant as well as other strains of the virus.
"A single dose of Pfizer or AstraZeneca was either poorly or not at all effective against the Beta and Delta variants," the authors concluded.
"Both vaccines generated a neutralising response that efficiently targeted variant Delta only after the second dose."
The French researchers also tested unvaccinated people who had survived a bout of COVID-19 and found their antibodies were four-fold less potent against the Delta strain, but a single dose of vaccine then dramatically boosted their antibody levels.
A previous study in the UK found two doses of Pfizer were 96 per cent protective against severe COVID-19 from the Delta variant and 88 per cent effective against symptomatic infection.
The Delta variant is spreading fast in parts of mainland Europe including Spain and France. The European Centre for Disease Prevention and Control (ECDC) estimates that this strain is 60 per cent more transmissible than earlier variants.
By August, the ECDC expects this variant to account for 90 per cent of all Covid-19 cases circulating in the European Union.