Attenuated vaccines can be produced by passing a virus through hundreds of different cell cultures and animal embryos. This process weakens the virus’s ability to reproduce to the point that it is unable multiply enough to harm the human host when administered in a vaccine. However, the immune system can still recognize the foreign pathogen and produces antibodies to attack the antigen. The body remembers the virus and is now able to defend itself from the same pathogen should it encounter it again. However, viruses in attenuated vaccines can occasionally change back into a disease-causing form or mutate into an even more virulent strain. Though this is highly improbable in most cases, oral polio vaccine has occasionally mutated and given paralytic polio to some patients.

 

Killed or inactivated vaccine, on the other hand, are completely unable to replicate and thus cannot become virulent again. The pathogen is treated with heat or chemicals like formaldehyde to destroy its reproductive capabilities, although it can still be recognized by the immune system. They are usually less permanent than attenuated vaccines and may require boosters for long-term immunity.

 

Toxoids do not contain an inactivated pathogen but rather an inactivated toxin secreted by the disease-causing bacteria, as the toxin is what causes the symptoms. Tetanus, for example, is caused the neurotoxin tenanospasmin which is produced by the Clostridium tetani bacteria.

 

Subunit and conjugate vaccines are vaccines which contain only parts of the target pathogens. Subunit vaccines are produced by using a protein isolated from the pathogen as the antigen. They can also be manufactured with genetic engineering. Genes coding for the vaccine protein is inserted into another virus or a producer cell. The protein is replicated when the virus reproduces, and the immune system recognizes the protein and builds up defenses against the disease. Vaccines manufactured in this manner are called recombinant vaccines. The hepatitis B vaccine and human papilloma virus vaccine are both recombinant vaccines.

 

Conjugate vaccines are produced from parts of the bacteria coat. It consists of a carrier protein and coat components linked together. The piece of the coat cannot cause disease and wouldn’t illicit a strong immune response, but when joined with the carrier protein, it triggers a more powerful response and generates immunity against future infections. Vaccines for pneumococcal bacterial infections are conjugate vaccines.