The study focuses on the role of journalists, public health officials, government communication officers, and political leaders in health crisis communication during the COVID-19 pandemic. The qualitative data set consists of qualitative interviews with key individuals involved in health crisis communication during the COVID-19 pandemic in the four countries, conducted between May and October 2022. The interviewees were recruited from four main groups (a) journalists who covered the pandemic, (b) public health officials and other public health professionals who played important roles in communication processes, (c) communication officers in government and other institutions involved in health crisis communication, and (d) political leaders. The number of interviewees ranged from 20 (Brazil) to 14 (US, Poland and Serbia). In general, gaining access to journalists and public health officials was easier than gaining access to government officials and politicians. All interviews were semi-structured and followed the same basic protocol, which was adapted for different countries and different types of interviewees, reflecting the role played by each of them in the health crisis communication process. Most interviews were conducted remotely, audio-recorded and then transcribed. A small number of interviewees declined to be recorded, and one interviewee responded to questions in writing. All recorded interviews were conducted in local languages, transcribed and translated into English (with the exception of those conducted in Serbian, which remain in original language). During transcription, the interviews were anonymized, and the interviewees were assigned a unique number, preceded by country abbreviation. All interviews were carried out following the relevant research ethics guidelines and procedures for protection of human subjects at participating universities; interviewees were offered anonymity and gave informed consent. Participants in Serbia, Poland and Brazil provided consent in written form (in accordance with protocols set out by Loughborough University, UK), while in the US, oral consent was recorded (in accordance with the ethics requirements of the University of California San Diego). The quantitative dataset is based on a cross-sectional survey with nationally representative samples, conducted in the four countries between November and December 2022. The questionnaire was designed to explore the patterns of public attitudes and information-seeking behaviour in relation to the pandemic, with questions about attitudes towards measures, trust in institutions, media use, as well as COVID-related misinformation and conspiracy theories. The data was collected by means of an online survey, carried out in all four countries between November and December 2022 by Lightspeed (Kantar). The survey included a total of 5,000 respondents (in Brazil, N= 1,500; in Poland, N= 1,000; in Serbia, N= 1,000; in the United States, N= 1,500), stratified by quotas according to sex, age, geographic regions, and income. Weights were not used as the data had a strong match with the census profiles.Media serve as important sources of information about health, and their role increases during public health crises. The way media select and present information during a crisis can have a significant impact on public attitudes and behaviour; it can encourage social cohesion and compliance with public health measures, or alternatively saw division and distrust. The presence of populist leaders obstructs the capacity of media organisations to engage in effective health crisis communication. It fosters anti-elite sentiments, contributes to divisive media coverage, and thereby encourages polarized attitudes and distrust among citizens, making them more vulnerable to misinformation spreading through socio-digital networks. Given the growing appeal of populism globally, activities aimed at making our societies more resilient in the face of future pandemics need a better understanding of how populism affects health crisis communication. This project developed the first comprehensive, comparative study of health crisis communication in the context of populist politics, focusing on four countries that were led by populist leaders during the pandemic: Brazil, Poland, Serbia, and the USA . Research examined the impact of populism on four aspects of the pandemic communication circuit during COVID-19: government-led health crisis communication, media policy, media coverage, and public attitudes. A separate strand of research also considered how domestic public debate responded to China and Russia's pandemic geopolitics