Cohort and Matched-Cohort Studies

Observational analyses estimate vaccine effectiveness.

There are several studies of vaccine effectiveness in England and Scotland. There have also been analyses of the vaccine roll-out in Israel.

Despite having similar names, vaccine effectiveness is a different concept to efficacy.

Vaccine efficacy is about reductions in disease. A trial compares vaccinated volunteers to a control group. The trial places people into groups at random. Researchers estimate how well the vaccine does at reducing disease. Trials can under-represent or exclude some people. That affects translation from participants to population.

Vaccine effectiveness is about the programme. How do vaccine programmes affect health outcomes, outside of trial conditions? Those outcomes include people going to hospital and death.

Vaccine effectiveness is about how the programme helps people. (Image: WHO)

Observational analyses are different to randomised trials. There is no experiment to establish. The data comes from collection systems.

Observational analyses can suffer statistical problems. Hidden variables can influence both the ‘cause’ and ‘effect’, distorting the studied associations. In statistical terms, these variables are confounders. Statistical issues can induce systematic errors in estimates.