Rabies is a viral disease that affects the central nervous system and is almost always fatal once symptoms appear. However, rabies can be prevented through vaccination even after exposure to the virus. Here is an explanation of why rabies vaccines work after infection:
How Rabies Infection Occurs
Rabies is caused by the rabies virus. It is transmitted through the saliva of infected animals, usually via a bite. The rabies virus travels from the wound site to the peripheral nerves and then makes its way to the central nervous system, where it causes inflammation of the brain and spinal cord. This leads to symptoms like aggression, excessive salivation, paralysis, and death.
After a bite from a rabid animal, the rabies virus binds to nerve endings at the wound site and travels through axons (long extensions of nerve cells) towards the central nervous system. This process is relatively slow, taking weeks or even months. The lengthy incubation period provides a window of opportunity for post-exposure vaccination.
How Rabies Vaccines Work
Rabies vaccines work by prompting the body to produce antibodies against the rabies virus. Here are the main types of rabies vaccines:
- Killed (inactivated) vaccines – These contain inactivated versions of the rabies virus. They cannot cause rabies.
- Live vaccines – These contain weakened (attenuated) versions of the rabies virus that stimulate immunity but do not cause disease.
- Recombinant vaccines – These contain genetically engineered rabies proteins. They do not contain any virus particles.
All types of rabies vaccines work by training the immune system to recognize the rabies virus and mount a response if exposure occurs. After vaccination, antibodies are produced that can neutralize the rabies virus, preventing it from entering the nervous system.
Why Post-Exposure Vaccination Works
Post-exposure rabies vaccination works because of the relatively long incubation period of the disease. After a bite, it takes time for the virus to travel to the central nervous system where it can cause lethal disease. Vaccination during this period stimulates an immune response capable of neutralizing the virus before it reaches the brain and spinal cord.
Some key reasons why post-exposure rabies vaccination is effective include:
- The rabies virus travels slowly through nerve axons at a rate of about 2-5 mm per day.
- It typically takes 1-3 months after a bite for rabies symptoms to appear, sometimes even longer.
- Vaccines can stimulate an immune response within days of administration.
- Rabies antibodies have time to reach sufficient levels and circulate through the body before the virus reaches the central nervous system.
This means there is a window of opportunity where vaccination can enable the immune system to clear the virus before it causes lethal effects in the brain and spinal cord.
Post-Exposure Vaccination Protocol
For effective post-exposure vaccination against rabies, quick medical attention is important. The recommended protocol includes:
- Immediate and thorough washing of the wound with soap and water to reduce virus particles.
- Injection of rabies immune globulin near the wound site to provide passive antibodies.
- A course of 4 doses of rabies vaccine given over 14 days, which stimulates active antibody production.
This combines both passive antibodies and active immunization to confer rapid protection against rabies infection after exposure. When administered promptly and properly, post-exposure vaccination is nearly 100% effective at preventing rabies.
Factors that Influence Efficacy
Some key factors influence the efficacy of post-exposure rabies vaccination:
- Site of the bite – Bites on highly innervated regions like the face and hands result in more rapid viral entry into nerves.
- Severity of the bite – Deep bites deliver larger viral loads that reach nerves faster.
- Time elapsed – The longer the delay before starting vaccines, the less time antibodies have to respond.
- Immune status – Weakened immunity can impair antibody response to vaccines.
That’s why bites on the head and torso or those that severely tear flesh have a poorer prognosis compared to milder injuries on the extremities. Seeking urgent medical care to begin vaccination is critical.
Limits of Post-Exposure Vaccination
While post-exposure rabies vaccination is highly effective, it does have limitations:
- If symptoms appear, vaccination may no longer be protective since the virus has already entered the nervous system.
- The severely immunocompromised may not mount a robust response even when vaccinated after exposure.
- In rare cases, the rabies strain may be incompatible with vaccines available.
- In remote regions, accessing vaccines promptly after exposure may not be possible.
Therefore, while post-exposure vaccination offers an excellent chance of preventing rabies, the protection is not absolute. That is why pre-exposure vaccination is recommended for people at high risk of rabies contact, such as veterinarians and animal handlers.
Conclusion
Post-exposure rabies vaccination leverages the relatively slow progression of the virus after a bite to stimulate immune defenses. Antibodies induced by vaccines have sufficient time to reach high levels in the body and neutralize the virus before it enters the central nervous system, where it becomes fatal. When administered promptly and properly, post-exposure prophylaxis is nearly 100% effective at preventing rabies. However, it is not a substitute for pre-exposure vaccination in people at occupational risk of rabies exposure.