**2. Methods**

Parallel surveys were undertaken in Italy, Latvia, and the UK. In Italy, a questionnaire (Supplementary Material) was distributed to 104 adult patients living in the north of the country, with the support of the Association of Patients CFS Onlus, which has an important role in assisting and supporting medical research and in disseminating knowledge of the disease [8]. The questionnaire had several sections. The first section sought general information (age, gender, education, place of residence, etc.), the second section addressed clinical history, and the third focused on the socio-economic consequences of the disease, including restrictions on daily life, sources of assistance, and understanding and awareness of the disease. The final section sought information on health status, reliance on physicians, the possible causes of illness, and future expectations. Quality of life was assessed using the instrument EuroQol-5D [9–11]. The patients were also asked to rate their quality of life in a scale from 0 to 100, where 100 represents the best imaginable QoL and 0 the worst, for the year before onset of illness, and for the year immediately preceding completion of the survey.

In Latvia, the patients' questionnaire (Supplementary Material) has been designed in accordance with the questionnaire prepared by the Italian team of Working Group 3 (socioeconomic), employed in the European program COST Action 15111 EUROMENE, in order to ge<sup>t</sup> a comparable data. The sample has included 75 valid observations, performed by 62 women and 13 men. Simultaneously, a questionnaire for GPs was distributed with support of the Latvian Association of Rural Family Doctors, taking into account that this association represents GPs working in urban and rural areas. The survey had included 20 questions, mostly on the criteria for granting a diagnosis, laboratory examinations, the involvement of specialists, and the methods of treatment. There were 91 completed responses from GPs.

To obtain comparison data from the UK, an internet survey was set up using the facility 'Smartsurvey'. A link to the questionnaire was circulated on 19 October 2020 via the internet group 'LocalME', with a request for UK residents with medically diagnosed ME/CFS to respond by 31 October. 448 questionnaires were completed by the deadline. The survey was structured in order to replicate as much as possible the Italian original questionnaire, though some variations were inevitable because of differences in the ways in which healthcare services are delivered. For the international comparison report we have calculated 95% confidence intervals for most of the parameters examined.
