A measure of how an intervention, vaccine or treatment, works once it has been authorised and used in the “real world” when it is given to the general  population.

Effectiveness is a term in epidemiology. It is a “real world” measure of how a vaccine or treatment which has already proven to have efficacy reduces disease in a population. It can also assess the balance of benefits and adverse events of an immunisation programme, not just the vaccine itself. Effectiveness is assessed under more natural or everyday conditions rather than in a controlled clinical trial. Vaccine effectiveness is proportional to vaccine efficacy, but it is also affected by how well target groups in the population are immunised,   which itself may reflect difficulties in maintaining proper storage conditions of a vaccine, such as the cold chain, access to health care, and vaccine cost.

An approved COVID-19 vaccine has not yet been rolled out to the general population in any country. Once a vaccine candidate is approved and rolled out, effectiveness studies can begin.

TIP: Don’t confuse efficacy with effectiveness. While they both measure the performance of a vaccine or treatment, they are measured in different settings.  


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EpidemiologyAdverse Event Following Immunisation (AEFI)VaccinesCOVID-19