While large research studies are underway, overall it appears that Guillain-Barre syndrome is a rare but serious phenomenon possibly linked to COVID-19. Given that more than 10.7 million* cases have been reported for COVID-19, there have been 10** reported cases of COVID-19 patients with Guillain-Barre syndrome so far.
It is certainly possible that there are other cases that have not been reported. The Global Consortium Study of Neurological Dysfunctions in COVID-19 is actively underway to find out how often neurological problems like Guillain-Barre syndrome is seen in hospitalized COVID-19 patients.
Also, just because Guillain-Barre syndrome occurs in a patient diagnosed with COVID-19, that does not imply that it was caused by the virus; this still may be a coincident occurrence. More research is needed to understand how the two events are related.
Due to the pandemic and infection-containment considerations, diagnostic tests, such as a nerve conduction study that used to be routine for patients with suspected Guillain-Barre syndrome, are more difficult to do. In both U.S. cases, the initial diagnosis and treatment were all based on clinical examination by neurological experts rather than any tests. Both patients survived but with significant residual weakness at the time these case reports came out, but that is not uncommon for Guillain-barre syndrome patients. Though the reported cases of Guillain-Barre syndrome so far all have severe symptoms, this is not uncommon in a pandemic situation where the less sick patients may stay at home and not present for medical care for fear of being exposed to the virus. This may skew our current detection of Guillain-Barre syndrome cases toward the sicker patients who have to go to a hospital.
*10.7 million cases worldwide as of 2nd July 2020
** The Lancet reported 19 cases of GBS July 2020