Vaccines are drugs that are given to people to make them immune to certain diseases. They contain the bacteria or virus that causes illness and disease or parts of the bacteria or virus.
The bacteria or virus is included in the vaccine so that the immune system can be taught to recognize and produce antibodies against it if a person is naturally exposed to it without experiencing any symptoms of illness or disease.
By stimulating the immune system to attack the agent, a vaccine can provide active immunity against a specific harmful agent. The antibody-producing cells, known as B cells (or B lymphocytes), remain sensitized and ready to respond to the agent if it ever enters the body after being stimulated by a vaccine.
A vaccine can also provide passive immunity by supplying antibodies or lymphocytes that have already been produced by a human or animal donor. Vaccines are usually given as injections. Vaccines applied to mucosal surfaces, such as those lining the gut or nasal passages, appear to stimulate a stronger antibody response and may be the most effective way to administer vaccines.
Scientists choose which type of vaccine to develop based on a variety of factors. Vaccines are available in a number of forms, including:
- Inactivated vaccines
- Live-attenuated vaccines
- Messenger RNA (mRNA) vaccines
- Subunit, recombinant, polysaccharide, and conjugate vaccines
- Toxoid vaccines
- Viral vector vaccines
A dead version of the disease-causing germ is used in inactivated vaccines.
Inactivated vaccines rarely provide the same level of immunity (protection) as live vaccines. These vaccines can be safely given to people who have a compromised immune system. However, a person with a weakened immune system may not experience the same level of protection as a healthy person who receives the vaccine.
As a result, you may need several doses over time (booster shots) to maintain disease immunity.
Vaccines that are inactivated are used to protect against:
- Hepatitis A
- Flu (shot only)
- Polio (shot only)
Live vaccines use a weakened (or attenuated) version of the disease-causing germ.
These vaccines elicit a strong and long-lasting immune response because they are so similar to the natural infection they help prevent. Most live vaccines can provide lifetime protection against a germ and the disease it causes with just one or two doses.
This vaccine works by allowing a virus or germ to reproduce long enough for the body to produce memory B-cells, which are cells that can recognize and remember a virus and generate an immune response against it for years after their initial response.
Live vaccines, on the other hand, have some disadvantages. People with weakened immune systems, long-term health problems, or those who've had an organ transplant should speak with their health care provider before receiving them because they contain a small amount of the weakened live virus.
They need to be kept cool, so they don't travel well. That means they can't be used in countries where refrigerators are scarce.
Live vaccines are used to protect against the following diseases:
- Measles, mumps, rubella (MMR combined vaccine)
- Yellow fever
Messenger RNA vaccines—also called mRNA vaccines
For decades, scientists have studied and worked with mRNA vaccines, and some of the COVID-19 vaccines were created using this technology. mRNA vaccines produce proteins to elicit an immune response. mRNA vaccines have several advantages over other types of vaccines, including shorter manufacturing times and no risk of disease in the person receiving the vaccine because they do not contain a live virus.
mRNA vaccines are used to protect against the following diseases:
Subunit, recombinant, polysaccharide, and conjugate vaccines
Specific pieces of the germ—like its protein, sugar, or capsid—are used in subunit, recombinant, polysaccharide, and conjugate vaccines (a casing around the germ).
These vaccines produce a strong immune response that is targeted to specific parts of the germ because they only use specific pieces of the germ. They can also be used on almost anyone who requires them, including those with compromised immune systems and long-term health issues.
One drawback of these vaccines is that you may need booster shots in the future to remain disease-free.
These vaccines are used to protect against the following diseases:
- Hib (Haemophilus influenza type b) disease
- Hepatitis B
- HPV (Human papillomavirus)
- Whooping cough
- Pneumococcal disease
- Meningococcal disease
Toxoid vaccines use a toxin (harmful product) produced by the disease-causing germ. Instead of the germ itself, they create immunity to the parts of the germ that cause disease. That is, the immune response is directed at the toxin rather than the entire germ.
Booster shots, like some other types of vaccines, may be required to maintain disease protection.
Vaccines against toxoids are used to protect against:
Viral vector vaccines
Virus vector vaccines have been studied for decades. Virus vector technology was recently used in some Ebola vaccines, and several studies have focused on viral vector vaccines for other infectious diseases like Zika, flu, and HIV. COVID-19 vaccines were also developed using this technology.
Viruses such as influenza, vesicular stomatitis virus (VSV), measles virus, and adenovirus, which causes the common cold, have all been used as vectors. One of the viral vectors used in some COVID-19 vaccines in clinical trials is an adenovirus. Vaccines against viral vectors are used to protect against:
There are four types of vaccines that are widely used to prevent a variety of diseases, but new vaccines are being developed that may be less expensive and provide longer immunity than existing vaccines.
Some existing vaccines, such as the live-attenuated vaccine, will have more side effects than others, making them unsuitable for people with long-term health conditions or weakened immune systems.
While some vaccines use live versions of a virus or bacteria to elicit an immune response, others can use only a portion of the virus or bacteria to do so, which may result in a stronger immune response against the germ due to its specificity.