Natural infections with varicella, tetanus and diphtheria have each been associated with Bellís Palsy. Thus, varicella, tetanus and diphtheria vaccines prevent Bellís Palsy by protecting against these natural infections. Vaccines currently routinely recommended to the general population in the U.S.* have not been shown to cause Bellís Palsy.
The only vaccine ever confirmed to cause Bellís Palsy was Berna Biotechís Nasalfluģ, an inactivated intranasal influenza vaccine adjuvanted with
E. coli heat-labile toxin which is no longer being produced. This vaccine was licensed for the 2000-2001 flu season in Switzerland and then permanently withdrawn from the market upon detection of the Bellís Palsy caused by the vaccine . It was never used in the United States.
The 2012 report by the Institute of Medicine (IOM) , now called the National Academy of Medicine (NAM), described two studies with sufficient validity and precision that both reported no association between inactivated influenza vaccine and Bellís Palsy [3, 4]. The report also described one study assessing an association between acellular pertussis vaccination and Bellís Palsy ; however, this study did not provide convincing evidence due to a lack of validity and precision . Most studies published since the 2012 IOM report have also reported no association between vaccination and Bellís Palsy [6, 7]; however, one study did find a temporal association between receipt of meningococcal conjugate vaccine concomitantly with other vaccines and Bellís Palsy .
Known causes of Bellís Palsy include infections due to Borrelia burgdorferi, the agent of Lyme disease, and zoster virus in Ramsay-Hunt syndrome. Infections with
Clostridium tetani or Corynebacterium diphtheria have been associated with facial nerve palsy as well, albeit very rarely . Although other viral infections such as herpes simplex virus (HSV) and varicella zoster virus (VZV) have also been associated with Bellís Palsy [9-12], the pathogenesis of Bellís Palsy remains poorly understood. Hypotheses include reactivation of latent viral infections in facial nerve ganglia  or an autoimmune mechanism possibly with segmental demyelination . Regarding the association of Bellís Palsy with Nasalfluģ, an influenza vaccine adjuvanted with E. coli heat-labile toxin, the most likely hypothesis is that the
E. coli enterotoxin resulted in inflammation and entrapment of the facial nerve in the facial canal [15, 16].
The IOM concluded that the only mechanistic evidence for an association between Bellís Palsy and tetanus or diphtheria vaccines was knowledge about the natural infection, and that there was no mechanistic evidence for hepatitis A, hepatitis B and influenza vaccines causing Bellís palsy.
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