Effectiveness of COVID-19 vaccines over time prior to Omicron emergence

A CITF-funded study published in Open Forum Infectious Diseases and led by Dr. Jeffrey Kwong (University of Toronto) on behalf of the Canadian Immunization Research Network (CIRN), highlighted that – prior to Omicron – the effectiveness of two doses of COVID-19 vaccine decreased over time against infection but remained high against severe outcomes over 11 months (January to November 2021).