Vaccine Safety: Blood-brain Barrier and Vaccines

Vaccine Safety: Blood-brain Barrier and Vaccines

Some people concerned about vaccine safety wonder whether vaccines may enter the brain and cause neurological conditions. In order to affect the brain, vaccine components would need to reach the brain. Importantly, our brains are protected by a barrier, called the blood-brain barrier (BBB), which keeps most foreign substances from entering the brain.

What is the blood-brain barrier?

The BBB is a membrane the separates the brain from the rest of the body. The BBB is composed of cells that are closely linked together, forming “tight junctions.” These junctions prevent substances from passing between cells and into new locations, such as the brain. As a result, to access the brain, substances must pass through, or interact directly with, cells that form the BBB. In this manner, cells of the BBB act as a gate. They limit which substances gain access to the brain, protecting it from pathogens and other harmful substances. But they still allow necessary nutrients to enter the brain and keep it healthy.

  • 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.
Transcript Transcript

What can cross the BBB and enter the brain?

The BBB is very restrictive, limiting what can access the brain. Glucose and other nutrients needed to keep the brain healthy and functioning properly are allowed to cross the BBB and enter the brain.

Typically, cells of the immune system are not able to cross the BBB. This helps prevent brain inflammation. Unfortunately, sometimes, pathogens damage the BBB. When this happens, those pathogens can leave the blood and enter the brain. Immune cells can also enter the brain because of the damage and they do so in an attempt to kill the invading pathogens. When this happens, the person may experience:

  • Meningitis – Inflammation of the outer membrane of the brain and spinal cord
  • Encephalitis - Inflammation of the brain

Meningitis and encephalitis are serious conditions that typically require hospitalization and may lead to permanent brain damage or death.

Different types of germs can cross the BBB and lead to serious infections:

  • Some bacteria that enter the bloodstream can cross the BBB. These bacterial infections are rare, but very dangerous. Examples can include Haemophilus influenzae type b, pneumococcus and meningococcus.
  • Viruses more commonly cross the BBB. While they, too, are dangerous and can be fatal, viral meningitis is usually less severe than bacterial meningitis. Some examples of viruses that can cross the BBB include Japanese encephalitis virus (JEV), mumps, varicella (chickenpox) and rubella. In addition, some viruses can infect the cells of the brain, which is more dangerous. One example is measles virus. When measles infects the brain, it can cause a fatal condition called sub-acute sclerosing panencephalitis (SSPE).
  • Fungi and parasites can also infect the brain, but this happens more rarely than bacterial or viral infections.

How do we know vaccine components don’t cross the BBB?

Vaccines contain versions of a pathogen that are different than the ones that cause disease. Many vaccines do not contain live pathogens, but rather they only include pieces of pathogens. When only pieces of a pathogen are present, they cannot replicate and, therefore, they cannot cause infections or damage to the BBB. Live, weakened vaccines replicate a very limited number of times, so the tissue damage that can result during an infection does not occur. Therefore, the BBB is not damaged by vaccines; instead, vaccines lead to protective immune responses that prevent infections and the damage that infections can cause.

Some people wonder if other components of vaccines can enter the BBB:

  • Aluminum salts -Many vaccines contain aluminum salts to boost the immune response to the vaccine. When aluminum salts enter the bloodstream, most are bound by a protein, called transferrin, and filtered out of the body by the kidneys. While more than half of the quantity of aluminum that enters the body is removed within 24 hours of injection, and 75% within two weeks, small amounts of aluminum, can remain in the body. This is true whether the aluminum was introduced by a vaccine or by food. Most aluminum that remains in the body accumulates in the bones, but about 1% of it is estimated to accumulate in the brain. While this sounds scary, we know in whom that is most likely to occur. Most often, health issues that involve aluminum are experienced by people with two characteristics. First, their kidneys are not working well or at all. Second, they have been exposed to high levels of aluminum over a long period of time (like months or years). In these situations, health issues have been associated with the bones or brain. We do not see these conditions in children who receive the routinely recommended vaccines. Questions have arisen about the role of aluminum in brain-based diseases, like Alzheimer’s. Although studies have found aluminum in brain tissue from deceased individuals with some of these diseases, it is unclear whether the aluminum is present because of the physical damage to the cells of the BBB or if it contributed to disease progression.
  • Other components found in vaccines - Stabilizers, like gelatin, are not likely to cross the BBB.

All vaccines go through heavy scrutiny before licensure and continue to be evaluated for safety after they are licensed.

How can I protect my child from pathogens that cross the BBB?

While anyone could find themselves in a situation of having an infection that leads to meningitis or encephalitis, these conditions are rare. However, people can protect themselves in a few ways:

  • Vaccination – Vaccines can protect against several infections that can cause meningitis or encephalitis, including measles, mumps, rubella, chickenpox (varicella), meningococcus, Haemophilus influenzae type b (Hib), pneumococcus, and Japanese encephalitis virus.
  • Prevention of insect bites – Since some insects carry infections that can cause encephalitis or meningitis, taking steps to decrease the chance of being bitten can help.
  • Practice good hygiene – Limiting exposure to any pathogen is useful in decreasing the chance of infection, including those that can lead to meningitis or encephalitis.

If you are concerned that your child is suffering from meningitis or encephalitis, contact your healthcare provider immediately.

Reviewed by Paul A. Offit, MD, on May 12, 2025