The word “vaccine” originates from the Latin Variolae vaccinae (cowpox), which Edward Jenner demonstrated in 1798 could prevent smallpox in humans. Today the term ‘vaccine’ applies to all biological preparations, produced from living organisms, that enhance immunity against disease and either prevent (prophylactic vaccines) or, in some cases, treat disease (therapeutic vaccines). Vaccines are administered in liquid form, either by injection, by oral, or by intranasal routes.
Vaccination is the administration of agent-specific, but relatively harmless, antigenic components that in vaccinated individuals can induce protective immunity against the corresponding infectious agent. In practice, the terms “vaccination” and “immunization” are often used interchangeably.
Vaccines are composed of either the entire disease-causing microorganism or some of its components.
They may be constructed in several ways
• From living organisms that have been weakened, usually from cultivation under sub-optimal conditions (also called attenuation), or from genetic modification, which has the effect of reducing their ability to cause disease;
• From whole organisms that have been inactivated by chemical, thermal or other means;
• From components of the disease-causing organism, such as specific proteins and polysaccharides, or nucleic acids;
• From inactivated toxins of toxin-producing bacteria;
• From the linkage (conjugation) of polysaccharides to proteins (this increases the effectiveness of polysaccharide vaccines in young children)
In addition to combining several serotypes of a disease- causing organism in a single vaccine (e.g. 13-valent pneumococcal conjugate vaccine), vaccines against different disease-causing organisms can be combined to provide protection against several different diseases. These combination vaccines may contain different types of vaccines. Combination vaccines against different diseases such as diphtheria, tetanus, pertussis, Heamophilus influenzae type b, Hepatitis B, and polio, are commonly used in childhood immunization schedules. These vaccines incorporate both viral and bacterial vaccines and contain toxoids, purified protein sub-unit vaccine, conjugated polysaccharide vaccine, recombinant protein vaccine, and inactivated viral vaccine respectively
Vaccines may also contain antigens against several types (or serotypes) of the same disease-causing organism, providing protection against each type. Polio and influenza vaccines each protect against 3 types of virus, and some bacterial vaccines like pneumococcal vaccine protect against up to 23 different serotypes of Streptococcus pneumoniae.
Types of vaccines
- Live-attenuated e.g. Measles, Mumps, Rubella, Varicella zoster
- Inactivated e.g. Hepatitis A, Influenza, Pneumococcal polysaccharide
- Recombinant sub-unit e.g Hepatitis B
- Toxoid e.g. Tetanus, Diphtheria
- Conjugate polysaccharide-protein e.g Pneumococcal, meningococcal, Haemophlius influenzea type b (Hib)
What does a vaccine contain?
In addition to the bulk antigen that goes into a vaccine, vaccines are formulated (mixed) with other fluids (such as water or saline), additives or preservatives, and sometimes adjuvants. Collectively, these ingredients are known as the excipients. These ensure the quality and potency of the vaccine over its shelf-life. Vaccines are always formulated so as to be both safe and immunogenic when injected into humans. Vaccines are usually formulated as liquids, but may be freeze-dried (lyophilized) for reconstitution immediately prior to the time of injection.
Preservatives ensure the sterility of the vaccine over the period of its shelf-life. Preservatives may be used to prevent contamination of multi-dose containers: when a first dose of vaccine is extracted from a multi-dose container, a preservative will protect the remaining product from any bacteria that may be introduced into the container. Or, in some cases, preservatives may be added during manufacture to prevent microbial contamination. Preservatives used in vaccines are non-toxic in the amounts used and do not diminish the potency of vaccines. But not all preservatives can be used in all vaccines. Some preservatives will alter the nature of some vaccine antigens.
How do vaccines work?
When inactivated or weakened disease-causing microorganisms enter the body, they initiate an immune response. This response mimics the body’s natural response to infection. But unlike disease-causing organisms, vaccines are made of components that have limited ability, or are completely unable, to cause disease
The components of the disease-causing organisms or the vaccine components that trigger the immune response are known as “antigens”. These antigens trigger the production of “antibodies” by the immune system. Antibodies bind to corresponding antigens and induce their destruction by other immune cells
The induced immune response to either a disease-causing organism or to a vaccine configures the body’s immune cells to be capable of quickly recognizing, reacting to, and subduing the relevant disease-causing organism. When the body’s immune system is subsequently exposed to a same disease-causing organism, the immune system will contain and eliminate the infection before it can cause harm to the body
The effectiveness and the duration of the protective effect of a vaccine depend both on the nature of the vaccine constituents and on the manner in which they are processed by the immune system. Some disease-causing organisms, such as influenza, change from year to year, requiring annual immunization against new circulating strains.
Which diseases are vaccine-preventable?
Smallpox was the first vaccine-preventable disease. After Edward Jenner’s publication on the use of cowpox to protect against smallpox, the practice of smallpox vaccination became increasingly widespread. But about 100 years would elapse until the development of a second human vaccine, Louis Pasteur’s rabies vaccine.
The development of new vaccines then grew exponentially, with several new human vaccines being introduced in the first half of the 20th century, but even more becoming available in the latter half and in the early 21st century. An intense period of innovation at the end of the 20th century led to the development of several new methods of producing vaccines, including the expression of proteins in recombinant organisms, the conjugation of polysaccharides to carrier proteins, and the construction of viral-like particles
The rapid growth in vaccine development is expected to result in more new vaccines becoming available within the next decade.
In theory, any infectious disease might be preventable with a vaccine. But a limited understanding of the immune mechanisms involved, and the highly variable nature of the immune response to each specific disease-causing organism, have meant that the development of vaccines has so far been limited to a number of viral and bacterial diseases. For some diseases, such as AIDS, vaccine development is particularly challenging because the HIV virus escapes the body’s natural immune response. For parasitic disease, complex life-cycles, or relatively large size, may limit the ability of vaccines to work effectively.
Even when immune mechanisms for specific diseases are understood, there is no guarantee that a same vaccine design can be successfully applied to other similar disease agents. For many years, scientists have been unable to develop safe and effective vaccines against diseases like respiratory syncytial virus (RSV)—a very common childhood respiratory infection—or dengue fever (a mosquito-borne disease that about 2.5 billion people are at risk of catching).
How efficacious are vaccines?
Vaccine efficacy varies according to the type of vaccine and the manner in which the vaccine antigen is processed by the immune system. Vaccine efficacy may also vary between different populations. However, in general, the efficacy of licensed vaccines ranges from above 70% to almost 100%. In other words, vaccines could be expected to reduce the attack rates in the vaccinated population by 70-100% compared to the attack rates in the unvaccinated population.
How safe are vaccines?
The benefits of vaccination are indisputable. Immunization has had one of the greatest impacts on health, second only to clean drinking water. Vaccines prevent death, illness and / or disability. But because of the immune reactions that they induce, vaccines can cause some discomfort.
The vast majority of adverse events associated with vaccines are minor and transient. These are typically pain at the injection site, or mild fever.
More serious adverse events occur rarely. Some serious adverse events may be so rare that they occur only once in millions of vaccine doses delivered, and some serious adverse events may occur so rarely that their risk cannot be accurately assessed. Some individuals may be sensitive to some components or trace elements in some vaccines, such as eggs, antibiotics, or gelatin. Otherwise, the cause of rare or very rare adverse events is usually unknown. It is believed that rare and very rare adverse events are associated with individual differences in immune responses.