Vaccines, how they work

Who I am
Philippe Gloaguen
Author and references

In 1796 an English doctor, Edward Jenner, decided to experimentally prove a theory about smallpox that had already been around for some time in medical circles. This disease was killing millions of people at the time. However, there was a bovine form of smallpox which manifested itself as pustules on the hands but which was essentially harmless. Many had noticed that those who milked the cows and contracted the bovine form of the disease, then did not get sick from the human one, much more serious and devastating.

Jenner decided to take some pus from the cowpox vesicles and inject it into a little boy through a small cut on his arm. The boy fell ill with cowpox and recovered promptly. At this point Jenner injected him with material coming from pustules of human smallpox patients: the little one showed no signs of the disease.

Although this experiment today is considered absolutely unethical, since it put the life of the unwitting son of Jenner's neighbor at risk, the birth of vaccines (whose term comes from 'cow') is due to this intuition.

The last case of smallpox occurred in 1977 and therefore the disease is considered to have been completely eradicated. For this reason the smallpox vaccine is no longer administered.

Diseases that do not recur

Jenner acted on the basis of a principle well known today: there are infectious diseases which, once contracted, guarantee immunity for life. Basically, they don't take it twice. This happens because the organism, if exposed again to the infectious agent, be it virus or bacterium, will recognize it and be able to fight it.

Vaccines are capable of 'teaching' the body to defend itself without causing the actual disease, introducing into the body only some parts of the infectious agent, precisely those that awaken the immune response. In other cases, however, viruses or bacteria are attenuated, that is, made less virulent by laboratory manipulations.

A vaccine may therefore contain elements of the infectious agent (parts of the capsule or particular proteins), the dead whole infectious agent or its attenuated form. The immune system identifies these substances, known as antigens, as foreign and produces particular proteins that circulate in the body and which are called antibodies. It is these that fight infections and neutralize antigens. Antibodies are produced by type B white blood cells (lymphocytes), whose main purpose is precisely to defend against infections.

Two types of vaccine: live attenuated and inactivated ones

Live attenuated vaccines contain a less active infectious agent that can reproduce no more than twenty times in the body. To get an idea, just know that a virus that is not inactivated reproduces itself thousands of times. However, this type of vaccine can cause a mild form of the disease in a number of people: in the case of chickenpox, for example, some scattered blisters will appear.

To prepare these vaccines, scientists isolate the infectious agent from a sick person, then grow it in a test tube. They take it out, check for changes and put it back in another tube. Sometimes even 80 passes are made before having a virus so accustomed to the protected environment of the test tube that it is almost incapable of replicating in humans, and therefore suitable to become the basis of the vaccine.

In inactivated vaccines, however, the infectious agent is killed, usually with a chemical such as formaldehyde. Some parts of the virus or bacterium, the most powerful from the antigenic point of view (ie the ability to awaken the immune system) form the basis of the vaccine.

Since the immune response to inactivated vaccines is typically weaker than that obtained with a live attenuated vaccine, such vaccines require multiple boosters. The advantage over a live attenuated vaccine is that there is no risk of developing the disease, even in a mild form. The only serious side effect that can occur is therefore the allergic one.

An obligatory choice

The choice of the type of vaccine does not always depend on the ability of scientists, but often on the characteristics of the disease. Most infectious viruses require live attenuated vaccines, while bacteria are easier to make inactivated vaccines. There are some exceptions, for example the rabies vaccine. Rabies is a viral disease, but the virus that causes it is too dangerous to administer in an attenuated form. Fortunately, an effective inactivated form has been developed.


In addition to the antigen, vaccines contain other substances necessary to make the product stable and safe, which is often transported very far from the place of manufacture. All of these substances, known as "excipients", must pass the same safety tests as the vaccine itself. Despite this, it is possible that an individual is allergic to a specific excipient.


Why are there no vaccines for all infectious diseases? The answer is complex, but the main reason is that some viruses, such as HIV for example, constantly change some of their characteristics. These mutations make the creation of a vaccine complex, which will easily be rendered useless by the continuous change in the characteristics of the infectious agent.


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