You probably know that vaccines prevent life-threatening diseases. You may also know that some vaccines are only given to children, (i.e. Chickenpox and MMR – Measles, Mumps, and Rubella) whereas others require you to make repeat visits to the doctor’s office through adulthood, for ‘boosters’ (i.e. TdaP – Tetanus, Diphtheria, and acellular Pertussis).
Why are there different timelines for different vaccines? Why do some protect better than others? Why do some need boosters?
To answer these questions, we need to discuss the different types of vaccines that exist.
Here’s a great introductory video from Ted-Ed on the immune system that introduces the basics of the vaccine types:
Here’s what all vaccines have in common:
Vaccines “teach” your body how to defend itself when a pathogen (such as a virus or bacteria) invades. They do this by exposing you to a small, safe amount of a weakened or killed pathogen. This exposure allows your immune system to make antibodies, proteins specifically targeting a pathogen to tell your body to fight off this foreign invader. Once your body has created these antibodies, it produces an immune system ‘memory’ of sorts, that recognize and attacks that pathogen if exposed in the future. If you encounter a pathogen after receiving the relevant vaccine, your body can react quickly to prevent illness.
Now, let’s discuss the differences:
There are four main types of vaccines:
- Live-attenuated vaccines
- Inactivated vaccines
- Subunit, recombinant, polysaccharide, and conjugate vaccines
- Toxoid vaccines
Live vaccines use a weakened (attenuated) form of the virus. A common technique to attenuate pathogens is to grow them in cells in which they do not reproduce very well. Growing in a hostile environment can weaken the pathogens, and as each generation adapts to the new environment, they become weaker with respect to their natural host, humans. Eventually, they are so well adapted to the hostile environment that they no longer produce disease in humans. These are the pathogens used in vaccines. Because these vaccines are so similar to the natural infection that they help prevent, they create the strongest, longest lasting immune response. For most live vaccines, 1 or 2 doses confer lifelong protection.
- Measles, mumps, and rubella (MMR) vaccine
- Varicella (chickenpox) vaccine
- Yellow Fever vaccine
- Smallpox vaccine
- Rotavirus vaccine
While live vaccines provide the best immune response, they have some limitations:
- Because they contain weakened live virus, some individuals with weakened immune systems (i.e. HIV) should discuss with their doctor before receiving them.
- They need to be kept cool to preserve their effectiveness – this makes their use difficult in very poor countries with limited access to cooling options.
Inactivated (killed) vaccines
Inactivated vaccines use the killed version of the germ that causes a disease. Rather than attenuated, this version is chemically processed to be completely inactivated (via heat, radiation, or chemical processing). As a result, these vaccines don’t provide as strong immunity as live vaccines. Many inactivated vaccines thus require booster shots to maintain immunity over long periods of time.
- Hepatitis A
- Flu (shot only)
- Polio (shot only)
Subunit, recombinant, polysaccharide, and conjugate vaccines
Subunit vaccines use part of a pathogen to provoke a response from the immune system (usually a specific surface protein).
Recombinant vaccines are essentially genetically engineered subunit vaccines, where a gene coding for a vaccine protein is inserted into another virus. When this ‘carrier’ virus reproduces, the vaccine protein is created and recognized by the immune system.
Polysaccharide vaccines are a type of subunit vaccines composed of long chains of sugar molecules on the surface of certain bacteria.
Conjugate vaccines are made using a combination of bacterial coatings, which are linked to a carrier protein. This combination is used as the vaccine. These vaccines create a more powerful immune response because the bacterial coating on its own would not generate a strong enough immune response.
All the vaccines listed above can be recognized by the body to make antibodies to fight future infection, but usually do not provide immunity that’s as strong as live vaccines. This is why you might need booster shots to maintain immunity.
- Hib (Haemophilus influenzae type b) disease
- Hepatitis B
- HPV (Human papillomavirus)
- Pertussis (Whooping cough, part of the DTaP vaccine)
- Pneumococcal disease (PCV)
- Meningococcal disease (Meningitis vaccines)
Toxoid vaccines contain a toxin or chemical made by the bacteria or virus that causes disease. These vaccines do not make you immune to the infection itself, but rather to the harmful effects of the infection. Toxins can be inactivated by treating them with formalin (a solution of formaldehyde and sterilized water). These safe, treated toxins are called toxoids. These vaccines yield a lower immune response, and usually require an adjuvant, which is a chemical substance that boosts the immune response, to generate immunity. Again, these vaccines may not confer lifelong immunity, and you will need booster shots for ongoing immunity (or after exposure to the potential toxin).
- Tetanus toxoid
This is not an exhaustive explanation of the differences between vaccine types, but hopefully helps understand the four major types of vaccines which exist today. For more detail, check out the References below.