On conducting diary-based social contact surveys to inform transmission models of close-contact infections: a systematic review and expert-informed questionnaire

Background: Diary-based social contact surveys are widely used to parameterise transmission models of close-contact infections. However, variation in study design, contact definitions, and questionnaire content limits comparability across studies. These challenges became particularly evident during the COVID-19 pandemic, when rapid and repeated data collection was urgently needed, often without harmonised instruments. To address these challenges, we aimed to develop an expert-informed questionnaire and update the recommendations on conducting diary-based social contact studies previously proposed by Hoang et al. (2019). Methods and findings: We conducted a systematic review of diary-based social contact surveys published between 31 January 2018 and 25 September 2025 using PubMed and the Web of Science Core Collection, extracting information on study design, survey methodology, contact definitions, and survey items from available questionnaires. Methodological quality and reporting were assessed using an adapted AXIS tool, with results summarised for each criterion. In total, we identified 129 studies across 62 countries, with most studies conducted during the COVID-19 pandemic (59.7% in 2021-2022). Retrospective reporting (86.0%) and non-probability sampling (77.5%) were largely adopted and became increasingly common during the pandemic, while non-paper-based data collection rose sharply from 2021 onwards. Across 36 retrievable questionnaires from general-population surveys, we identified 54 survey items. An online assessment involving 26 experts informed the classification of these items according to their perceived importance, resulting in an expert-informed questionnaire comprising 22 core items. Based on our findings, we updated previous recommendations on sampling, survey methods, contact definitions, behavioural change, essential contact information, and data governance. Conclusions: This study provides an expert-informed questionnaire and updated recommendations to support harmonised social contact data collection, improve comparability across studies, and strengthen preparedness for future public health emergencies.