A Single-Dose of COVID-19 Vaccine May Not Protect Us from the SARS-CoV-2 Delta Variant: An Evidence Review .

The Delta variant quickly emerged as a major global challenge, spreading faster than earlier strains and showing higher transmissibility than Alpha. Evidence now shows that neutralizing antibody activity and vaccine effectiveness drop against Delta, particularly after only one vaccine dose—a gap large enough to raise real concern. After two doses, protection remains solid, though still reduced compared with Alpha. This pattern has clear policy implications: delaying second doses is far riskier in the face of Delta, and accelerating full vaccination—especially for vulnerable groups—has become essential. At the same time, easing public-health measures too quickly could amplify transmission while immunity gaps remain.