The World Health Organization (WHO) follows a rigorous guideline development process to develop evidence-based policy and guide practice.¹ Central to this process is a rigorous, reproducible and comprehensive evidence synthesis, which serves as the foundation for guideline development. At the start of the pandemic, the WHO’s infection prevention and control (IPC) emergency interim guidance for COVID-19 took an expedited approach,² rapidly addressing emerging issues. Given the lack of direct evidence on IPC measure for COVID-19, guideline developers utilised indirect evidence from existing WHO guidelines on severe respiratory illness.^3,4 The guideline later transitioned to using standard WHO guideline development methodology¹ and using evidence directly addressing IPC measures or COVID-19, culminating in a comprehensive and consolidated guideline.^3,5

Transitioning to the standard guideline development process involved identifying the core set of questions that required evidence synthesis; of which 32 research questions were identified by the guideline steering committee and guideline development group (GDG) and 17 systematic evidence reviews commissioned (steering community and GDG composition can be found in the acknowledgement section of the latest addition of the infection prevention and control in the context of COVID-19: A guideline).^3,4,5 These reviews provided the basis for formulating recommendations through evidence synthesis and assessing the certainty of the evidence, with support of a methodologist. In addition, recognising the importance of contextual factors such as health and care workers’ values and preferences in developing recommendations, five qualitative evidence syntheses were performed to addresses these areas.³

Key questions were reviewed simultaneously within a constrained timeframe. Given the substantial research demand, multiple groups were commissioned to conduct reviews. Some have already been published,^6,7,8 while others are included in this supplement (Table 1 and Table 2). Additional systematic reviews were conducted independently and are not included here, but details are referenced in the guideline.⁵

The reviews followed standardised systematic review methods, such as searching multiple databases, applying predefined inclusion and exclusion criteria, assessing risk-of-bias and determining evidence certainty using Grading of Recommendations, Assessment, Development and Evaluation (GRADE).¹ Evidence from randomised controlled trials was often either non-existent or very limited; consequentially, the reviews were broadly inclusive of various study designs including non-randomised/non-comparative studies, observational, cohort, case-control and ecological studies. Bias and confounding were addressed as part of the GRADE process, often resulting in a judgement of low certainty evidence.

While most of the details of how the COVID-19 IPC guideline was developed are available within the methodology and supplemental material associated with the guideline,⁵ the articles in this theme collection offer an in-depth, complementary perspective into the evidence available to the guideline panel at the time of writing. Amid the evolving landscape of the COVID-19 pandemic, balancing responsiveness with emerging IPC issues while reliably appraising the emerging evidence through systematic reviews proved challenging. These articles also mark a milestone in the guideline development process – a shift from the reliance on studies with indirect and/or very low certainty evidence and expert opinion to direct, robust evidence and standard methodologies.³ Furthermore, this themed collection highlights the research gaps and underscores the need for high-quality studies to establish a robust evidence base for future IPC guidelines in the context of respiratory infectious disease outbreaks/pandemics.⁴ Publishing these articles promotes transparency, provides a detailed record of the process, and serves as a reference for future adaptation. This will allow us to revisit and build upon the evidence as needed, should a future threat require the rapid development of guidelines, just as was done for COVID-19.^3,4