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Different diseases require different types of vaccines. Types of vaccines include:

• Live vaccine. Also known as attenuated (weakened) vaccine because it contains live versions of a virus or bacterium that have been weakened so as not to cause disease in the body.

A live vaccine produces a very strong immune system response and often protects people for life. However, there is a small possibility that the weakened microbe introduced through the vaccine could mutate into a disease-causing microbe. For this reason, live vaccines are unsuitable for people with impaired immune systems, such as people with HIV/AIDS or cancer. Examples of diseases that can be prevented by a live vaccine include:

• • Measles
  • Mumps
  • Rubella
  • Yellow fever
    
    
• Inactivated vaccine. When making an inactivated vaccine, scientists kill the disease-causing microbe while maintaining its structure. This produces an immune system response against the disease, although this response may not be as strong as for a live vaccine. Thus, booster shots may be needed to maintain immunity.

Inactivated vaccines are safer and more stable than live vaccines because there is no chance that the dead microbe will mutate into a disease-causing microbe. This makes inactivated vaccines more suitable for people with weakened immune systems. Examples of diseases that can be prevented by an inactivated vaccine include:

• • Influenza
  • Polio
  • Rabies
    
    
• Subunit vaccine. Unlike live and inactivated vaccines, which use the entire microbe structure, a subunit vaccine is composed only of the antigens of the microbe. Antigens are the molecules on a microbe that stimulate an immune system response. A subunit vaccine contains fewer antigens than vaccines that contain the entire microbe. This reduces the risk of the body reacting adversely to the vaccine. Examples of diseases that can be prevented by a subunit vaccine include:
  
  
• • Hepatitis B
  • Whooping cough (pertussis)
  • Streptococcal pneumonia
    
    
• Toxoid vaccine. Some bacteria secrete a toxin that may be more harmful to the body than the bacteria themselves. For diseases caused by a bacterial toxin, a toxoid vaccine containing inactivated toxins may be used. Examples of diseases that can be prevented by a toxoid vaccine include:
  
  
• • Diphtheria
  • Tetanus
    
    
• Conjugate vaccine. A conjugate vaccine is a special vaccine developed for infants. Some microbes are covered by sugar molecules that help hide them from the immune system. Infants and young children are especially at risk from these microbes because their immune system is immature and may be unable to recognize or respond to the microbe.

The conjugate vaccine combines toxins and antigens that the child’s immune system will recognize (along with the associated sugar molecules) in order to produce an immune system response. The child’s immune system can then respond to the sugar molecules present on the disease-causing microbes. Examples of diseases that can be prevented by a conjugate vaccine include:

• • Haemophilus influenzae type b (Hib) disease
  • Streptococcal pneumonia
    
    
• DNA vaccines. These types of vaccines include the genetic structure (particularly the genes that code antigens) of disease-causing microbes without introducing the microbe itself. By doing this, the vaccine can make the body produce antigens and stimulate an immune system response.

These types of vaccines have the potential to be effective in preventing illnesses such as HIV and malaria because they generate a strong antibody response without the risk of using a live microbe. They are also cheap and easy to produce. Scientists are still experimenting with DNA vaccines and, currently, there are no DNA vaccines approved by the Food and Drug Administration (FDA) for use in humans.

• Recombinant vector vaccines. These vaccines are still being researched, although they appear to have the potential to prevent many types of illnesses. Recombinant vector vaccines are similar to DNA vaccines, except that they use a harmless microbe shell (known as the vector, or vehicle of transmission) to introduce the genetic material from a disease-causing microbe to the human body. Thus, a harmless microbe mimics a harmful microbe, producing a strong immune system response. A weakened microbe may also be used as the vector in this type of vaccine.