What is Covid 19 vaccine efficacy?
Vaccine efficacy is the percentage reduction in a disease in a group of people who received a vaccination in a clinical trial. It differs from vaccine effectiveness, which measures how well a vaccine works when given to people in the community outside of clinical trials.
Volunteers taking part in vaccine clinical trials often undergo close monitoring. The trial team is usually aware of the participants’ general health and any relevant health conditions.
Participants usually report any side effects and may fill out daily symptom monitoring diaries.
Many clinical trials have exclusion criteria such as pregnancy, particular health conditions, and age. Trials involving experimental vaccines rarely include children or seniors until scientists have collected a significant amount of safety data to protect these groups from potential harm.
Vaccine efficacy only provides information about how well a vaccine works under the conditions of the clinical trial. Scientists usually base it on factors that they can quantify, such as numbers of laboratory-confirmed cases of COVID-19.
But the ideal conditions of a clinical trial do not necessarily reflect what is happening in the real world outside of clinical trials.
Vaccine effectiveness tells us how well a vaccine works under real-world conditions once people outside of clinical trials receive the vaccine.
Many factors can influence how a vaccine performs outside of clinical trials. One of these is the health of those receiving the vaccine. Underlying health conditions can affect vaccine effectiveness.
Another factor is how the disease-causing pathogen changes with time. The viruses that cause the flu are prone to mutations that make vaccines less effective. Vaccine developers update the flu shot every year to try to achieve a good match to the most prevailing seasonal flu strains.