Live, attenuated vaccines
Attenuated vaccines can be made in a variety of ways. Some of the most common methods include the passage of the disease-causing virus through a series of cell cultures or animal embryos (usually chicken embryos). Using chick embryos as an example, the virus develops in different embryos in a row. With each pass, the virus becomes better in breeding in chick cells but loses its ability to replicate in human cells. A virus intended for use in a vaccine can be developed through “passage” through over 200 different embryos or cell cultures. Eventually, the attenuated virus will be unable to reproduce well (or not at all) in human cells and can be used in a vaccine. All methods involving the passage of a virus through a non-human host produce a version of the virus that can still be recognized by the human immune system but can not be reproduced well in a human host.When the resulting vaccine virus is given to a human, it will not be able to replicate enough to cause disease but will still cause an immune response that can protect against future contamination.
One concern to be taken into consideration is the possibility that the vaccine virus will return to a form capable of causing illness. Mutations that can occur when the vaccine virus is replicated in the body can lead to more virulent strains. This is very unlikely, as the vaccine virus reproduction capacity is limited. However, it is taken into account when developing an attenuated vaccine. It is worth noting that mutations are somewhat common with oral polio vaccine (OPV), a live vaccine that is consumed instead of an injection. The vaccine virus can mutate into a virulent form and lead to rare cases of paralytic polio. Therefore, OPV is no longer used in the United States and has been replaced by the Inactivated Poliomyelitis Vaccine (IPV) in the Adult Childhood Immunization Program.
Protection from a live, attenuated vaccine usually exceeds that provided by a dead or inactivated vaccine.
Damaged or inactivated vaccines
An alternative to attenuated vaccines is a dead or inactivated vaccine. Vaccines of this type are generated by inactivating a pathogen, typically using heat or chemicals such as formaldehyde or formalin. This destroys the pathogen’s ability to replicate, but it keeps it “intact” so the immune system recognizes it. (Generally, it is used “inactivated” rather than “killed” to refer to vaccines of this type, because viruses are generally not considered alive).
Because deadly or inactivated pathogens can not be reproduced at all, they can not revert to a more infectious form capable of causing disease (as discussed above with live, attenuated vaccines). However, they tend to provide less protection from live vaccines and are more likely to require enhancers to create long-term immunity. Vaccinated or inactivated vaccines in the US Childhood Vaccination Program include the inactivated poliomyelitis vaccine and the seasonal influenza vaccine (in shot form).
ArchedSome bacterial diseases are not directly caused by the bacterium itself, but by a toxin produced by the bacterium. An example is the tetanus: its symptoms are not caused by the Clostridium tetani bacterium, but by a neurotoxin it produces (tenanospasmin). Immunizations for this type of pathogen can be done by inactivating the toxin that causes symptoms of the disease. As with organisms or viruses used in dead or inactivated vaccines, this can be done by treatment with a chemical such as formalin, or using heat or other methods.Immunizations generated using inactivated toxins are called toxoids. Arthropods can actually be considered as dead or inactivated vaccines, but sometimes they are given their own category to highlight the fact that they contain inactivated toxin and not inactivated form of bacteria.
Toxic immunizations in the United States’ recommended childhood immunization program include tetanus and diphtheria immunizations, which are available in a combined form.
Subunit and conjugate vaccines
Both subunits and conjugate vaccines contain only patches of pathogens they protect.Subunit vaccines only use part of the pathogen target to elicit an immune response. This can be done by isolating a particular one.