Exponential growth, high prevalence of SARS-CoV-2, and vaccine effectiveness associated with the Delta variant

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections were rising during early summer 2021 in many countries as a result of the Delta variant. We assessed reverse transcription polymerase chain reaction swab positivity in the Real-time Assessment of Community Transmission-1 (REACT-1) study in England. During June and July 2021, we observed sustained exponential growth with an average doubling time of 25 days, driven by complete replacement of the Alpha variant by Delta and by high prevalence at younger, less-vaccinated ages. Prevalence among unvaccinated people [1.21% (95% credible interval 1.03%, 1.41%)] was three times that among double-vaccinated people [0.40%(95% credible interval 0.34%, 0.48%)]. However, after adjusting for age and other variables, vaccine effectiveness for double-vaccinated people was estimated at between similar to 50% and similar to 60% during this period in England. Increased socialmixing in the presence of Delta had the potential to generate sustained growth in infections, even at high levels of vaccination.

Automated summary

During early summer 2021, the Real-time Assessment of Community Transmission-1 (REACT-1) study in England found that infections were rising due to the Delta variant, with unvaccinated individuals having three times the prevalence of infection compared to double-vaccinated individuals. Despite vaccination, the vaccine effectiveness for double-vaccinated individuals was estimated to be around 50% to 60% during this period. Increased social mixing in the presence of the Delta variant had the potential to generate sustained growth in infections, even with high levels of vaccination.