In individuals who had not previously been infected with SARS-CoV-2, a single dose of either the Pfizer or the AstraZeneca vaccine induced a barely detectable level of neutralizing antibodies against the Delta variant. About 10% of the sera neutralized this variant. However, a two-dose regimen generated high sero-neutralization levels against the Alpha, Beta and Delta variants in individuals sampled at week 8 to week 16 after vaccination. Levels of neutralizing antibodies are highly predictive of immune protection from symptomatic SARS-CoV-2 infection25. A recent report analysing all sequenced symptomatic cases of COVID-19 in England was used to estimate the effect of vaccination on infection26. Effectiveness was notably lower for the Delta variant than for the Alpha variant after one dose of the AstraZeneca or the Pfizer vaccine.
The two-dose effectiveness against the Delta variant was estimated to be 60% and 88% for the AstraZeneca and the Pfizer vaccine, respectively26. Our neutralization experiments indicate that antibodies elicited by the Pfizer and AstraZeneca vaccines are efficacious against the Delta variant, but about three- to fivefold less potent than they are against the Alpha variant. There was no major difference in the levels of antibodies elicited by the Pfizer or the AstraZeneca vaccines.