There is a very small possibility that some components or some vaccines cross the hematoencephalic barrier. This does not imply they are not safe
A website which is self defined as Catholic has echoed from the results of a guide published by an organisation known as America’s Frontline Doctors (AFLDS) in which is is claimed that vaccines against covid-19 cause alleged brain damages, among them “chronic inflammation and thrombosis”, and that this may contribute to tremors, chronic lethargy, stroke, Bell’s paralysis and symptoms of amyotrophic lateral sclerosis (ALS). This is MISLEADING: while it is true that a small portion of the Spike protein which is generated after vaccination can move to other body tissues circulating through the bloodstream in mice and arrive to the hematoencephalic barrier —placed in the encephalus—, but not cases have been notified in humans, and the likelihood for this to happen is “extremely low”, according to the experts.
"The Spike protein, together with the lipid nanoparticles, have the ability to cross the hematoencephalic barrier, which offers a special protection for these sensitive areas of the body. The risks of such a penetration include chronic inflammation and thrombosis(coagulation) in the neurologic system, which promotes tremors, chronic lethargy, cerebrovascular accident (stroke), Bell’s paralysis and ALS-like symptoms"
There is a possibility —very small, but real— that some components or some vaccines cross the hematoencephalic barrier, which is placed between the brain blood vessels and the cerebral tissues, and which protects the brain from many harmful substances. This does not imply they are not safe.
In the case of vaccines against covid-19, the theoretical risk that the S protein crosses this barrier also exists, “Some studies in mice indicate that the S protein (the S1 region) can cross the hematoencephalic barrier when such a fraction is directly injected in the bloodstream”, Adelaida Sarukhan, an immunologist and science writer at the Institute for Global Health (ISGlobal, in Catalan) in Barcelona, indicates to Verificat.
Vaccines are intramuscular
However, as the immunologist clarifies, vaccines are intramuscular and not intravenous. Moreover, the Spike protein coded by the mRNA vaccines or viral vector are “transmembrane”, that is, it is not released inside the extracellular space, but it remains attached to the membrane of the muscle cells”. This “eases the immune system to see it” and it also prevents it from moving freely to other body organs, she adds.
Hence, it is quite improbable that significant quantities of the S protein ended up freely circulating in the bloodstream —having the risk to end up in other organs and/or in the hematoencephalic barrier— indicated the immunologist. In fact, trials with mRNA vaccines coding for luciferase —a protein that emits light— have been done in mice, or marked with a radioactive isotope: “In the rest of the body tissues tissues less than a 1% of the protein is detected, according to what EMA indicates in this document, and what is released to the bloodstream is being degraded in the liver, as is it explained in this Science journal blog.
In fact, for the moment thrombotic events have not been associated with mRNA vaccines after hundreds of millions of administered doses in the general population, and the rare cases of venous thrombi associated with AstraZeneca and Johnson&Johnson vaccines seem to be given by other vaccine components which have nothing to do with the S protein.
What is indeed more possible is that the viruses themselves cross the hematoencephalic barrier. It has been observed in cases with infections for the viruses causing Japanese encephalitis and mumps, as well as for SARS-CoV-2 —at least in mice— according to a study published in Nature Neuroscience. In this investigation, scientists observed that the S or Spike protein crossed such a boundary, being then absorbed by several brain regions.