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What is a Vaccine

Vaccines
11 August 2025

Vaccines are one of the main strategies we have to significantly reduce the risk of catching serious and often dangerous illnesses. They are a form of safe and preventive medicine, helping us protect our and our children's health. To learn how vaccines work, we must first understand the immune system.

How the immune system works:

When bacteria or viruses invade the body, they multiply and attack it. This invasion is called infection, and it often develops into an illness. To fight infection, the immune system activates various types of white blood cells, which protect the body and “remember” the pathogen so it will fight it efficiently and promptly in subsequent encounters.

How vaccines work: Vaccines introduce the body to disease-causing viruses and bacteria, training the immune system to respond swiftly to the invasion and thus preventing the disease.

Types of vaccines:
  • Inactivated vaccine: This vaccine contains a virus or bacteria that cannot cause a disease, but the body learns to recognize them.
  • Live attenuated vaccine: This type of vaccine contains either a virus or bacteria that has been "attenuated," or weakened, so it cannot cause a disease.
  • Toxoid vaccine: This vaccine uses inactivated toxins so they cannot cause a disease.
  • Subunit vaccine (also called a recombinant or acellular vaccine): This vaccine contains small parts of the virus’s or bacteria’s envelope.
  • Conjugate vaccine: This vaccine contains a component of the virus or bacteria, such as proteins, sugars, or the capsule surrounding the pathogen, and not a whole virus or bacteria.
  • mRNA vaccine: This vaccine contains mRNA, a blueprint of part of the virus. The mRNA enters the cells and instructs them to produce a large amount of a specific viral protein, which fades away in time. The immune system recognizes the protein created in our body cells and mounts a response.

It is important to remember that vaccines cannot treat illnesses caused by a virus or bacteria you already have before vaccination; however, they can prevent future infections from the strains included in the vaccine.

Vaccines are one of the main strategies we have to significantly reduce the risk of catching serious and often dangerous illnesses. They are a form of safe and preventive medicine, helping us protect our and our children's health. Vaccines are considered one of the most life-saving interventions in the history of humanity. It is estimated that they save approximately 2.5 million lives each year worldwide. To understand their function, we must first understand how our immune system works and protects us from viruses and bacteria.

How the immune system works: recognize, attack, remember

When bacteria or viruses invade the body, they multiply and attack it. This invasion is called an “infection” and can often lead to disease. To fight infection, the immune system activates various types of white blood cells, each with specific roles in protecting the body:

Macrophages

These are white blood cells that act against antigens on the outer envelope of a virus or bacteria. The body recognizes these antigens as hazardous, and the macrophages engulf and digest the virus or bacteria.

B cells

These are white blood cells whose role is to produce specific antibodies against the invading virus or bacteria. The antibodies attack the antigens that the macrophages present.

T cells

These are white blood cells that attack infected body cells. T cells are also involved in maintaining the body’s immune memory.

Immune memory: The first time the body encounters a virus or bacteria that causes an infection, it takes several days for the immune system to produce the necessary response to fight the invaders and overcome infection. After an infection, the immune system “remembers” the strategies it used to defend itself against the disease. The body retains various T cells, known as “memory cells,” which are activated quickly each time the body encounters the same virus or bacteria. When the recognized antigens are encountered, the memory cells help activate the B cells, which quickly produce the appropriate antibodies to target them.

How vaccines work: A simulation of infection and the development of immune memory

Vaccines cause the body to recognize viruses and bacteria that cause diseases, teaching the immune system to respond quickly when exposed to them and preventing the development of diseases. Every disease has its vaccine, creating a simulation of the infection unique to the disease. The simulation does not cause a disease, but it does cause the immune system to produce memory cells (T cells) and specific antibodies, similar to the body’s response to the disease. Some vaccines provide lifetime protection, and in others, booster shots are needed to strengthen the memory cells that were created to maintain the protection for many years.

It takes approximately ten days for the body, after vaccination, to produce antibodies. Mild symptoms, such as fever, may sometimes occur after vaccination. These symptoms are normal and can be expected when the body builds immunity. Afterward, the body retains memory cells that will remember how to fight the specific disease upon future exposure.

For information about potential side effects after vaccination

NOTE

NOTE

Vaccines cannot treat illnesses caused by a virus or bacteria you already have before vaccination; however, they can prevent future infections from the strains included in the vaccine.

Types of vaccines

Routine vaccines administered to infants and children include the following six vaccine types:

It contains dead bacteria or viruses that can’t make you sick, helping the body learn to recognize them. This type of vaccine usually needs several doses to create and keep enough immunity.

Example: Inactivated polio vaccine.

This vaccine contains either a virus or bacteria that has been "attenuated," or weakened so they cannot cause a disease. This vaccine is similar to a natural infection, and in this way, it “teaches” the immune system. These vaccines are highly effective; however, they cannot be given to those with a weakened immune system as a result of immunosuppressive medications, including chemotherapy.

Example: The MMRV against measles, mumps, rubella, and chickenpox.

This vaccine prevents diseases caused by toxins that bacteria produce. During vaccine preparation, the toxins undergo an attenuation process (attenuated toxins are called toxoids), so they cannot cause a disease. When the immune system receives a toxoid vaccine it learns how to fight the natural toxins.

Example: The tetanus vaccine.

This vaccine contains small parts of the virus or bacteria’s envelope, not the entire virus or bacteria. Because these vaccines contain only the antigens necessary to stimulate the immune system, side effects are extremely rare.

Example: The pertussis vaccine (part of the DTaP vaccine, which also includes vaccines against diphtheria and tetanus).

This vaccine contains a component of the virus or bacteria, such as proteins, sugars, or the capsule surrounding the pathogen, and not a whole virus or bacteria. Therefore, these vaccines are considered highly effective.

Example: The pneumococcal vaccine.

This vaccine contains mRNA, a blueprint of part of the virus. The mRNA enters the cells and instructs them to produce a large amount of a specific viral protein, which fades away in time. The immune system recognizes the protein created in our body cells and mounts a response.

Example: The COVID-19 vaccine.

Active immunization and passive immunization

All the vaccines included in the routine vaccination plan for infants and children are active; namely, they cause the body to produce antibodies and create an immune memory. 

Besides them, there are also passive immunizations, which are ready-made antibodies that can fight the pathogen.

Passive immunizations are used in the following cases:

  • When an active vaccine cannot be used, for example, with pregnant women or those who are immunocompromised, or after exposure to an infection with certain diseases, such as measles.
  • When there has already been exposure to a fast-spreading disease or a dangerous toxin, and you can’t wait for the body to produce its own antibodies—for example, to prevent rabies after an animal bite.
  • When no active vaccine exists for a disease—for example, the RSV vaccine given to premature babies and infants.

Passive immunizations provide short-term protection and do not stimulate the body to produce antibodies and memory cells to fight the cause of disease next time. Therefore, all the routine vaccines are active.

A combined vaccine

A combined or a combination vaccine is a single vaccine that contains multiple vaccines within it. For example, the MMRV, containing vaccines against measles, mumps, rubella, and chickenpox, is administered in a single shot. In this way, it is possible to reduce the number of pricks that infants and children undergo and to advance as much as possible the protection of the body against diseases.

Booster shot

With active vaccines, the immune system's ability to create a response against the cause of disease may diminish with time, mainly because the number of memory cells in the body decreases. As a result, the immune system cannot recognize the pathogen quickly enough, and the immune reaction starts anew. Therefore, some vaccines may require an additional booster dose of the original vaccine to stimulate the immune system to act more effectively.

For example, vaccines against diphtheria, tetanus, and pertussis are given to infants at the Child Health Centers (Tipat Halav) during their first year. Booster shots are then given in the second and eighth grades at school. However, not every vaccine requires a booster dose. For example, the vaccines against Hepatitis A and Hepatitis B do not require booster doses.

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