The study has some limitations to note, some of which are raised in an accompanying editorial:
A relatively small number of people were tested, particularly in some of the experiments. The CFS samples all came from patients who had severe disability, prolonged disabling fatigue, cognitive defects and immune system abnormalities, and who were from regions where there were “outbreaks” of CFS. It is possible these patients are not representative of the full spectrum of patients with CFS, which can range in severity. The selection of cases that were clustered together in “outbreaks” could mean that these cases have a different cause or trigger from more isolated cases. The characteristics of the healthy people whose blood samples were used were not reported, and there may have been more differences from the CFS cases than simply the disease itself that contributed to the differing rate of XMRV infection. Although the researchers attempted to rule out contamination of their samples, the molecular biologist who co-discovered the XMRV virus suggests in the accompanying editorial that they did not do enough to completely rule out contamination. He also points out that confirmation of the results by an independent group blinded to whether the samples came from cases or controls is “vital”. Although the study suggested that the virus could spread to other cells in the laboratory from white blood cells or fluid from blood, this does not mean that the virus would necessarily be able to spread from person to person. Despite these limitations, the causes of CFS are not yet known and available treatments are limited, so these findings will be of much interest to the research community, doctors, and patients. Further research is needed to confirm these findings in more samples, and to establish whether the XMRV infection occurs before or after the onset of CFS.