Recent studies indicated that the effectiveness of the coronavirus disease 2019 (COVID-19) messenger RNA (mRNA) vaccines especially the mRNA-1273 (Moderna) and BNT162b2 (Pfizer-BioNTech) vaccines, against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was high for US veterans who were vaccinated in January and February of 2021.
However, the emergence of the Delta variant of concern during summer 2021 along with its increased transmissibility and virulence led to the waning of the COVID-19 vaccine effectiveness.
A new research letter published in JAMA Network Open assessed the effectiveness of the mRNA1273 and BNT162b2 vaccines among fully vaccinated male veterans who were 65 years of age or above from July to September 2021.
About the study
The study was a case-control one that was approved by the US Department of Veterans Affairs Medical Center Institutional Review Board in White River Junction, Vermont. The study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline where electronic medical data was obtained from the Veterans Health Administration Corporate Data Warehouse.
The observation time was divided into three periods, pre-Delta (before May 2021), rising Delta (May and June 2021), and high Delta (July to September 2021). Estimation of the vaccine effectiveness against SARS-CoV-2 was carried out along with the assessment of the association between SARS-CoV-2 infections and time since vaccination.
The results indicated that the pre-Delta vaccine effectiveness against SARS-CoV-2 was 94.5 percent in the first month after vaccination but reduced to 87.9 percent by three months. However, during the high-Delta period, the effectiveness of the vaccines was reported to be 62 percent in the first month that decreased to 57.8 percent by three months. Further decrease in vaccine effectiveness occurred in months five through seven.
The current study was therefore quite effective in determining the efficacy of the vaccines during the high-Delta period. The effectiveness was found to be high during the pre-Delta and rising Delta periods as compared to the high-Delta period. The only limitation of the study is that it focuses only on old veterans which could reduce the generalizability of these findings.