In March 2021 the World Health Organization (WHO) made interim guidance available on best practices for undertaking vaccine effectiveness (VE) evaluations in Evaluation of COVID-19 vaccine effectiveness: interim guidance, followed in July 2021 by an addendum addressing VE evaluations in the context of new variants of concern (VOC).

That guidance still generally applies to carrying out VE evaluations. Nonetheless, in the past year, several factors have arisen that indicate the need for a second addendum to the interim guidance, including lessons learned from the many VE evaluations already conducted to date, waning of protection, Omicron emergence, and the increasing proportion of the population with hybrid immunity. 

This addendum addresses some of the methodological aspects of VE evaluations that have been learned during the past year, as well as those that have become relevant in the current epidemiological setting of the COVID-19 pandemic. For some of the COVID-19 vaccine methodology issues there are still insufficient data to make a recommendation, in which case different options for approaching VE evaluations are presented.

Report by the World Health Organization