A large study carried out by the US Centres for Disease Control and Prevention shows that the efficacy of mRNA vaccines drops from 91% to 66% against the Delta variant.This is reported by Medical Xpress.
he study involved thousands of healthcare staff and other frontline workers across 6 states. Participants were tested once a week and tested again if they developed Covid-19 symptoms, thereby allowing researchers to determine the efficacy of mRNA vaccines against symptomatic and asymptomatic infection.
During the period of December 14, 2020 to April 10, 2021, before the highly-contagious Delta variant became the dominant strain, the vaccines demonstrated an efficacy of 91%. However, in the weeks that followed that period, up to August 14, that efficacy rate fell to 66%.
However, researchers have pointed out that some of this could be attributed to the fact that the protection offered by vaccines will decline over time anyway, and that the 66% rate was based on quite a short study period, involving fewer cases. They emphasise the ongoing importance of vaccination.
Meanwhile, the protection provided by two doses of the Pfizer/BioNTech and the Oxford/AstraZeneca vaccines starts to wane within six months, new research suggests.