Table 2.

Existing limitations of post-COVID-19 condition research and potential mitigation strategies

	Issue	Potential mitigation strategies
1.	The definition of the post-COVID-19 condition.	A few initiatives were launched, including a WHO working group aiming to provide a clinical case definition of the post-COVID-19 condition.
2.	Pathophysiological mechanisms still lacking.	A WHO working group has been set up to outline plausible hypotheses regarding the underlying immunological and physiological mechanisms of post-COVID condition. Multinational studies aimed to dissect the underling mechanisms of the post-COVID-19 condition should be lunched by multilateral organisations and universities.
3.	Rapidly emerging data.	Core Outcome Set (COS) should be developed keeping the balance between speed and quality. Acute COVID-19 COS initiatives [41] may be used as an example of efficient management and rapid development. Involve principle investigators from ongoing studies to investigate the possible additional sources of data and allow for the dissemination of the COS upon development. Close interaction and collaboration with the WHO to ensure global geographical coverage and worldwide applicability of the COS.
4.	Target population and scope: ● Hospitalised cohorts may be potentially different to those studies investigating non-hospitalised patients. ● Criteria for hospitalisation vary substantially within hospitals and countries (i.e. hospitalised patients are different). ● The need for a separate Post-COVID-19 condition COS development for children has not been established.	Despite the focus of COVID-19 research on adults, all age and severity (during acute phase) groups (including asymptomatic individuals) should be included (approaches to patient routing differ within and between the countries and criteria for hospital admission vary). It is imperative to develop COS for children and their carers as the post-COVID-19 condition may potentially have a detrimental life-long effect on child health. A single COS aiming at clinical as well as research settings may be developed.
5.	What to measure? ● There is a need to define which data should be assessed in the trials and in clinical practice.	Ongoing systematic reviews may assist with the development of a list of candidate outcomes for the evaluation as part of a Core Outcome Set. Patient engagement should drive the agenda to ensure that patient-important outcomes are captured. This can be achieved by survey/Delphi process and consensus meetings. Post-COVID-19 condition COS development is a priority.
6.	How to measure the Post-COVID-19 condition? ● Existing “validated” tools (e.g. quality of life instruments) have not been validated in COVID-19, and study participants are often asked about their experience pre-COVID retrospectively, which may lead to selection and recall bias. ● New measurement instruments may need to be developed if no adequate instruments exist for prioritised Core Outcome Domains.	Measurement instruments used in the studies should be systematically reviewed and assessed for validity/truth, discrimination ability, and feasibility.