Attenuated vaccine contains a less virulent form of a pathogenic organism that helps in the prevention of a disease. Let us have a look at it in detail.
Long ago, there was a very lethal disease called the smallpox plaguing people around the world. Ancient Chinese doctors found a method called ‘variolation’, wherein they infected a healthy person with minute scab powder from infected people with smallpox. The effects varied in individuals, like mild infection to even death. However, it was observed that most of the people who had been exposed to the smallpox scabs, did not develop an infection.
In the late 1700s, an English doctor was inspired by a story of a young milkmaid. She said she could never contract smallpox as she had suffered from a bout of cowpox. People who were or had been infected with cowpox, strangely never ever fell prey to smallpox. Thus, the English doctor took a bold step and purposely infected a young boy with cowpox. Once the young boy was cured of cowpox, the doctor infected him with smallpox. The boy did not contract smallpox. The doctor who performed this experiment was Edward Jenner, who bought forth to the world the process of vaccination. This bold experiment took a little time to be recognized by the scientific community. However, it helped people open their eyes to the benefits of vaccination.
Over the years, there have been many changes with the process of making vaccines. They tend to make us sick, but in a mild way and help our bodies build the required antibodies to prevent a full-fledged infection. There are three types of vaccines available. The first is the ‘killed’ or ‘inactivated’ type. The organism is killed and then introduced into the body of a healthy person. This helps in building antibodies against the organism. An example of killed version is the typhoid vaccine.
The second type is ‘acellular’ vaccine. This is produced using only the antigenic part of the organism. This antigenic part of disease-causing organism may be its flagella, capsule or protein part. The vaccine for Haemophilus influenza B (HIB) is an acellular type. Both, killed as well as acellular types do not cause disease or lead to any symptoms of the disease. The third one has the chances of developing infection in the vaccinated individual. This is what is called the attenuated vaccine.
These vaccines can be defined as ‘the use of live pathogenic organism that have been modified to produce a less virulent form, and which has the ability to stimulate the immune system to produce antibodies against the virulent form of the organism.’
The production of these vaccines is such that the pathogen tends to retain all its antigenic properties. This organism, however, can no longer cause disease in the person it is inoculated in. There are several methods for producing it. The first method was the one used by Jenner, where he used the cowpox organism to prevent smallpox. In the second method, one can grow the organism in a live culture that is an egg or tissue culture. This cultured organism is then introduced in small doses to a patient through inhalation.
The organism then tends to grow within the body of the patient and the immune system begins to develop antibodies against the organism. So, whenever the person encounters a fully pathogenic organism, the body is able to fight it off with the already present antibodies. There are several other methods that are used for vaccine production; like growing strains by mutagenesis or removal of virulent genes by target deletions in the DNA.
Low doses are administered to patients. The pathogenic organism in the vaccine has very low virulence. These organisms reproduce very slowly within the body. This is the reason why they are not given to immunocompromised individuals. This is because there are chances that the pathogen may revert to its virulent form.
There are quite a few advantages of an attenuated whole agent vaccine. Firstly, they activate all the phases of an immune system. They require less of booster doses as the organism tends to grow really slowly within the body. They provide quick immunity to the body. They are low in cost and are easy to transport as well as administer. Mostly, they are given through the oral route, thus, reducing the complications related to sterile injections.
There are a few ill effects of attenuated whole agent vaccine; like it has the ability of reverting back to its virulent form. Immunocompromised individuals may develop the disease due to the inoculation of live pathogens in their body. Also, one needs to maintain a constant temperature while transporting them.
- MMR (Measles, Mumps, and Rubella) vaccine
- Oral polio vaccine (OPV)
- Chickenpox vaccine
- Yellow fever vaccine
- Attenuated flu vaccine
- BCG vaccine
- Typhoid vaccine
These vaccines contain live organisms, but in a less virulent form. One or two dosages evokes cellular and antibody responses, and the immunity mostly continues for life.