(CNN) FDA warns of potential rare nerve complication with Johnson & Johnson coronavirus vaccine. Quoting,

“Today, the FDA is announcing revisions to the vaccine recipient and vaccination provider fact sheets for the Johnson & Johnson (Janssen) COVID-19 Vaccine to include information pertaining to an observed increased risk of Guillain-Barré Syndrome (GBS) following vaccination,”

Guillain-Barré Syndrome has infrequently been linked to vaccination, most famously with the 1976 swine flu outbreak. In clinical settings, unrelated to vaccination, one cause is well understood. Refer to (PubMed) Campylobacter jejuni infection in Guillain-Barré syndrome: a prospective case control study in a tertiary care hospital.  (full text here) Quoting,

The commonest implicated causative organism the world over is Campylobacter jejuni (C. jejuni).

C. jejuni is a bacteria.  The coat of campylobacter jejuni contains antigen sub sequences common with human nervous tissue. Antibodies created by the immune system cross-react with nervous tissue. This autoimmune response, not the bacteria itself, causes GBS. Presence of the bacteria itself in the vaccine is neither expected or required; the antigen sequences, molecular chains, are sufficient to provoke GBS.

The 1976 swine flu vaccine was cultured in eggs. In poultry, C. jejuni is endemic. Sterile eggs are required, but sterile-chain failures occurred with unknown frequency. C. jejuni was never isolated from the vaccine, though with the coincidence of mechanism and event, C. jejuni  contamination remains the leading hypothesis to this day.

In five-part Johnson & Johnson, Explanations for Clotting ?, I developed a hypothesis, left incomplete. Part 5 set the stage for unwritten Part 6: nucleation of platelets, with subsequent clot formation, around focal bacterial infections in the cerebral venous sinuses. But there was at that time no legitimate reason, surviving Occam’s Razor, to introduce bacterial infection. Since the literature does not support viral infection as a cause of platelet nucleation, Part 6 remained unwritten.

With GBS as a clue, there is enough to look at bacterial contamination of batches of PER.C6, the human cell line used to culture the adenovirus vector. If it exists, it won’t be  C. jejuni, which is far too conspicuous. This is not an easy job.

For a very few special cases, the light microscope you last saw in high school remains a clinical tool. But a microbial contaminant of PER.C6 would likely be too small and too fugitive to visualize except after the fact. The detection of novel pathogens remains a research topic.

Possible batch dependence of Astrazeneca clotting hints at a similar mechanism.

Takeaway: Explanations linking the J&J shot to  both CVS thrombosis and GBS could be linked by bacterial contamination. 

Pasteur: “Chance favors the prepared mind.”