We aimed to perform meta-analyses to summarize the overall effectiveness of the mRNA-1273 vaccine against COVID-19 caused by the Delta variant from real-world studies. A systematic literature search with no language restriction was performed in electronic databases to identify eligible observational studies that reported the effectiveness of the mRNA-1273 vaccine to prevent reverse transcription-polymerase chain reaction (RT-PCR) confirmed COVID-19 caused by Delta variant of SARS-CoV-2 (B.1.617.2). A random-effects meta-analysis model was used to estimate the pooled odds ratio (OR) at a 95% confidence interval (CI), and the vaccine effectiveness was indicated as (pooled OR − 1)/OR. Five studies were included for this systematic review and meta-analysis. The meta-analysis revealed that the administration of mRNA-1273 vaccine protected against RT-PCR confirmed COVID-19 caused by Delta variant ≥21 days post first dose, with pooled vaccine effectiveness of 66% (95% CI: 65%–67%), as well as ≥14 days after the second dose, with pooled vaccine effectiveness of 91% (95% CI: 84%–95%). In conclusion, the mRNA-1273 vaccine offers a substantial protection rate against RT-PCR confirmed COVID-19 caused by the Delta variant upon full vaccination, although with slightly reduced effectiveness relative to other strains of SARS-CoV-2.