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The SARS-CoV-2 omicron variant emerged in November 2021 and consists of several mutations within the spike. We used serum from mRNA vaccinated individuals to measure neutralization activity against omicron in a live-virus assay. Following 2-4 weeks after primary series of vaccinations, we observed a 30-fold reduction in neutralizing activity against Omicron. Six months after the initial two vaccine doses, sera from naïve vaccinated subjects showed no neutralizing activity against omicron. In contrast, COVID-19 recovered individuals six months after receiving the primary series of vaccinations show a 22-fold reduction with majority of the subjects retaining neutralizing antibody responses. In naïve individuals following a booster shot (3rd dose), we observed a 14-fold reduction in neutralizing activity against omicron and over 90% of subjects show neutralizing activity. These findings show that a 3rd dose is required to provide robust neutralizing antibody responses against the omicron variant.