November 17, 2021
By Brendan J. Kelly, MD, MS
A recently published report from Robilotti and colleagues in Clinical Infectious Diseases presents a single-center, retrospective cohort study of health care personnel who were tested for SARS-CoV-2 between November 2020 and August 2021, as the dominant variant in New York City transitioned from Iota/Alpha to Delta.
The cohort included fully vaccinated, partially vaccinated, and unvaccinated personnel. The authors compared vaccine effectiveness between the pre-Delta and Delta-dominant periods (i.e., before or after July 1, 2021), using the Centers for Disease Control and Prevention definition of “breakthrough infection” as detection of SARS-CoV-2 RNA in a respiratory or saliva specimen collected 14 days or more after receipt of the second dose of an mRNA vaccine. The authors calculated the rate ratio of cases per total person-days in each group (unvaccinated, partially vaccinated, or fully vaccinated) for each time period. They defined vaccine effectiveness as one minus the observed rate ratio. In the pre-Delta period, vaccine effectiveness was 94.5%; this fell to 75.6% in the Delta-dominant period. The authors noted that no hospitalizations occurred across vaccinated health care personnel during the entire study period, but 17 hospitalizations and one death occurred among unvaccinated personnel.
This observational study from New York City corroborates recent reports from the United Kingdom, including reports from Lopez Bernal et al. and from Pouwels et al., which show reduced effectiveness of both adenoviral-vector and mRNA vaccines against the Delta variant relative to the Alpha variant. Robilotti and colleagues conclude that COVID-19 mRNA vaccines demonstrate durable clinical protection with a moderate reduction in vaccine effectiveness against infections due to the Delta variant. They suggest that their findings support the continued development of vaccine booster strategies for health care personnel in the United States.