Can Vaccines Cross the Blood-Brain Barrier?

In this Science Made Easy video, Dr. Offit discusses the role of the blood-brain barrier and addresses concerns related to whether vaccines can cross this barrier.


Can vaccines cross the blood-brain barrier?

Paul Offit, MD: Hi, my name is Paul Offit. I am talking to you today from the Vaccine Education Center, here at the Children’s Hospital of Philadelphia. Parents are sometimes concerned about whether or not vaccines could cause harm, especially to the brain. I mean, is it possible that vaccines could enter the brain and cause developmental delays, or autism, or epilepsy, or attention-deficit disorder, or hyperactivity disorders. How good is the brain at keeping these vaccines away? The thing that’s critical in terms of the brain is something called the blood-brain barrier. Now, this is a semi-permeable membrane that exists between the circulation, meaning the blood, and the brain itself. So, it lets in certain things, like oxygen and glucose and things that brain cells need to grow, but it doesn’t let another things. So, for example, there are germs, pathogens, like viruses or bacteria, that can damage the blood-brain barrier and then allow those viruses or bacteria to enter. So, for example, measles virus can enter the brain and cause inflammation called encephalitis. Bacteria like Streptococcus pneumoniae, Haemophilus influenzae type b, meningococcus can also damage the blood-brain barrier, enter and cause meningitis. But the vaccines don’t do that. In healthy children, the vaccines don’t do that. The blood-brain barrier excludes those vaccines from entering and causing harm. So, I think people can be reassured by that.

So, I’ll give you another example. In your bloodstream, you have proteins, proteins like albumin; and you have about 7 grams or so per 100 milliliters of proteins like albumin. But if you look in the spinal fluid, you see a very, very low amounts of protein; you see maybe 20, 30 milligrams — so, not grams but milligrams, which is thousandths of a gram. So, that’s an example of how the blood-brain barrier excludes proteins from the brain, and proteins are much smaller than say viruses or bacteria. So, unless there is really damage caused by those germs, the vaccines that prevent those germs don’t enter the brain through the blood-brain barrier, because the blood-brain barrier excludes them. Thank you.

Related Centers and Programs: Vaccine Education Center