Magnus, Per, Gunnes, Nina, Tveito, Kari et al. · Vaccine · 2015 · DOI
This large study of the Norwegian population during the 2009 flu pandemic found that people who caught the H1N1 influenza virus were more than twice as likely to develop ME/CFS afterward. However, people who received the pandemic flu vaccine did not show an increased risk of developing ME/CFS. This suggests that actually getting infected with the flu virus itself, rather than the vaccine, may trigger ME/CFS in some people.
This study directly addresses a major concern among ME/CFS patients by examining whether the pandemic flu vaccine triggers CFS/ME. Using comprehensive national health data, it provides reassurance that the vaccine was not associated with increased CFS/ME risk while strengthening evidence that infectious triggers—specifically symptomatic influenza infection—may be involved in disease pathogenesis.
This study does not establish the mechanism by which influenza infection triggers CFS/ME, nor does it explain why only some infected individuals develop the condition. It cannot determine causation definitively—only association—and uses specialist health care diagnoses, which may underestimate true CFS/ME prevalence if many cases remain undiagnosed. The findings are specific to pandemic H1N1 and may not generalize to other influenza strains or non-influenza infections.
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
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