The following is the summary of “Effectiveness of MRNA booster vaccine among healthcare workers in New York City during the Omicron surge, December 2021 to January 2022” published in the December 2022 issue of clinical microbiology and infections by Robilotti, et al.

The purpose of this study is to compare the efficacy of 2-dose (2D) and 3-dose (3D) mRNA vaccinations in healthcare workers (HCWs) in the presence of Omicron variant dominance. The efficacy of 2D and 3D vaccine series against SARS-CoV-2 variants and the clinical outcomes of HCWs may inform return-to-work recommendations. SARS-CoV-2 infections among HCWs at a large tertiary cancer center in New York City were retrospectively examined from December 15, 2020, to January 15, 2022, to estimate infection rates (aggregated positive tests / person-days) and 95% CIs over the Omicron period in 3D and 2D mRNA vaccinated HCWs, and to compare these groups using rate ratios. 

Researchers outlined the signs and symptoms of infections after vaccination and the bearing of previous (pre-Omicron) COVID infection on the vaccine’s efficacy. 

By January 15, 2022, 12461 of the 20857 HCWs in our cohort had received a third dosage of the mRNA vaccine used in our study. During the Omicron dominating period of December 15, 2021, through January 15, 2022, the infection rate ratio for 3D versus 2D vaccinated HCWs was 0.667 (95% CI 0.623, 0.713), equating to an estimated 33.3% efficiency of the 3D vaccine compared to two doses only. After 3D + 14 days, 1,315 HCWs developed breakthrough Omicron infections. 

About 16% of 3rd-year medical students and 11% of 2nd-year medical students had no symptoms from their Omicron infection. This study shows that vaccinated individuals exposed to 3D mRNA during the Omicron surge were better protected from COVID-19 infection than those exposed to 2D mRNA. Furthermore, in contrast to 2D vaccination, the benefit of 3D immunization persisted even after controlling for previous COVID-19 infection status.