How CDC Monitors Vaccine Effectiveness
CDC uses a multilayered approach to understand how well COVID-19 vaccines work in the real world. This approach includes ongoing analyses of surveillance data to monitor impact of vaccines and studies of vaccine effectiveness.
Monitoring Vaccine Impact
CDC uses public health surveillance data to monitor rates of COVID-19 cases, hospitalizations, and deaths by vaccination status to help identify overall patterns or trends in the population level impact of vaccines. This information might also signal if there are any potential changes in how well vaccines work that can be further studied through more robust, controlled vaccine effectiveness studies.
Monitoring Vaccine Effectiveness
Vaccine effectiveness is a measure of how well vaccination protects people against infection, symptomatic illness, medically attended illness, including emergency department and urgent care visits, and severe illness, including hospitalization and death.
CDC conducts observational studies specifically designed to estimate protection provided by vaccination under “real-world” conditions. These vaccine effectiveness studies provide estimates that help us better understand how well the vaccines work in different groups of people, against different health outcomes (such as infection, symptomatic illness, hospitalization, and death), and during periods when different virus variants are circulating (such as Omicron and Delta). Results from these studies generate the evidence needed to inform COVID-19 vaccine policy decisions.
Vaccine Effectiveness Studies
CDC works with partners to study COVID-19 vaccine effectiveness using several data collection platforms and study designs. Vaccine effectiveness studies vary based on the outcome (e.g., infection, hospitalization) and population (e.g., residents of long-term care facilities, children).