What are Vaccines?

There are several diseases like Diphtheria, whooping cough (pertussis), measles, mumps, Yellow fever, small pox and German measles (rubella) that are unfamiliar to many these days. However, in the 19th and early 20th centuries, these illnesses struck hundreds of thousands of people worldwide and among these most were children. These illnesses killed tens of thousands of people. Today these diseases are all but forgotten. This change has happened largely because of vaccines.

Vaccine history

The term ''vaccine'' was derived from the Edward Jenner's 1796 use of the term ''cow pox'' (Latin ''variolæ vaccinæ'', adapted from the Latin ''vaccīn-us'', from ''vacca'' cow). He was the pioneer of using cow pox vaccines to prevent small pox infections.

Vaccines and the immune system

When an individual is vaccinated against a disease or an infection, say Diphtheria, his or her immune system is prepared to fight the infection. Once vaccinated when the person is exposed to the bacterium that causes it the body gears up to fight off the infection. This whole battle of the immune system with the invading bacterium is so rapid that most people do not observe or feel the infection at all.

Vaccines take advantage of the body’s natural ability to learn how to eliminate almost any disease-causing germ, or microbe, that attacks it. Once vaccinated the body “remembers” how to protect itself from the microbes it has encountered before.

What are vaccines made of?

vaccine is a biological preparation that improves immunity to a particular disease. Traditional vaccines contain either parts of microbes or whole microbes that have been killed or weakened so that they don’t cause disease.

When a person is inoculated with these preparations, the immune system confronts these harmless versions of the germs. The immune system quickly clears them from the body.

In turn the body remembers the germs so that later in life when it encounters the real live virulent germs it may be able to fight it off with the retained memory against the particular germ.

Types of vaccines

Some vaccines are prophylactic and are used to prevent or ameliorate the effects of a future infection by any natural or "wild" pathogen. Some vaccines however may also be therapeutic for example cancer vaccines that are being developed against cancer.

Vaccine Benefits

Once a person’s immune system is trained to resist a disease, the person becomes immune to it. Before vaccines, the only way to become immune to a disease was to actually get it and, with luck, survive it. This type of immunity against an illness is called naturally acquired immunity, wherein the person has to suffer the symptoms of the disease and also risk the complications, which can be quite serious or even deadly. In addition, if the disease is contagious it may also be passed on to family members, friends, or others who come into contact.

Vaccines, which provide artificially acquired immunity, are a much safer way to become immune. Vaccines can prevent a disease from occurring in the first place and also decrease the risk of complications and risk of transmission. It is much cheaper to prevent a disease than to treat it.

Vaccine schedules

Until recently, most vaccines were aimed at babies and children alone. Now more and more vaccines are developed for use among elderly, pregnant mothers, adolescents, travellers and adults in a population.

In addition, vaccines are increasingly being administered in form of combination of more than one component. Vaccinations of animals are being used both to prevent their contracting diseases and to prevent transmission of disease to humans.

Vaccine History

Vaccines have been found to be the most successful and cost effective public health measures that prevent disease and save lives. This is particularly true among children all over the world. Over the last half of the 20th century, diseases that were once all too common became rare in the developed world, due primarily to widespread immunization. Hundreds of millions of lives have been saved and billions of dollars in public health expenditures have been saved with widespread vaccinations.


In the 20th century alone, smallpox was responsible for an estimated 300 to 500 million deaths. In 1967, the World Health Organization (WHO) estimated that 15 million people contracted the disease and 2 million died that year. Since the 1970’s and 80’s small pox is completely eradicated from all parts of the world through the effective use of vaccines.


Polio is a severely debilitating illness that may leave a child paralyzed for life. In the years following World War II, polio was the most feared disease among parents in the United States.

In 1952, polio permanently paralyzed 21,000 people in the United States alone. With the development of vaccines against polio, the rates have gone down by more than 99 percent.

The fight to fully eradicate polio worldwide continues while cases are still being detected in four countries – Afghanistan, Pakistan, Nigeria and India (no cases reported for over one year now).


Measles is a very common and contagious viral illness of childhood. In children, particularly those with malnutrition, it can be dangerous. Measles can cause deafness, blindness, encephalitis, and death.

Between 2000 and 2008, measles deaths dropped by 78 percent worldwide due to immunization. However, more than 20 million people continue to be infected by measles each year, resulting in 164,000 deaths in 2008, primarily among children.

German measles or rubella

This is a relatively mild viral illness of childhood. However, it can cause severe birth defects in children born to mothers who contracted the disease in the early stages of pregnancy. This is called congenital rubella syndrome.

The introduction of a rubella vaccine in 1969 has greatly reduced the incidence of congenital rubella syndrome in the developed world. Worldwide though the disease still causes approximately 110,000 cases each year, and causes blindness, deafness, and mental retardation in thousands more.


Diphtheria was once a dreaded bacterial illness. In the 1920s, diphtheria infected an estimated 100,000 to 200,000 people per year in the United States and killed 13,000 to 15,000. It is now rare in the United States but is responsible for about 5,000 deaths each year in developing countries, primarily among children.


Pertussis, or whooping cough causes spasmodic, uncontrollable coughing that persists for weeks. Before the arrival of the vaccine, pertussis infected an average of 200,000 people a year in the United States alone. Rates have declined with the rise of vaccination against the infection but pertussis still kills almost 195,000 people every year.

History of vaccines

The first vaccine developed was against smallpox by Edward Jenner, English "country" physician, in Berkeley. He found that dairy maids with cow pox were relatively immune to small pox. He took the exudates and secretions from a cowpox pustule on the hand of dairymaid Sarah Nelmes and inserted it into the arms of an 8 year old boy James Phipps on May 14, 1796.

The vaccination was effective since the boy did not catch small pox even when he was infected with small pox virus six weeks after the vaccination. Jenner published his findings in 1798. Despite opposition, vaccination soon became accepted practice.

Louis Pasteur

Louis Pasteur generalized Jenner's idea by developing what he called a rabies vaccine (now termed an antitoxin), and in the 19th century compulsory vaccination laws were passed. The golden age of vaccine development did not come until after World War II, when several new vaccines were developed in a relatively short period. Their success in preventing diseases such as polio and measles changed the history of medicine altogether.

Immunization campaigns

In 1967, the WHO spearheaded a massive immunization campaign against smallpox. Within ten years, this disease had been vaccinated out of existence.

Wild-virus polio, which once circulated widely in nearly every region of the world, is now present in only a handful of countries, without a case diagnosed in the United States since 1979.

Measles, mumps, rubella, diphtheria, and pertussis were reduced from frightening epidemics to rare outbreaks within a few decades.

Types of vaccines

  • Killed vaccines - Vaccines containing killed microorganisms. This includes vaccines against flu, cholera, plague, polio and hepatitis A
  • Attenuated – the vaccine contains live organisms that have been weakened to disable their virulent properties. Examples include: Yellow fever vaccine, rubella, measles, mumps, typhoid, tuberculosis (Bacillus Calmette Guerin or BCG) etc.
  • Toxoids – These are inactivated toxic compounds secreted by the organism. Examples include Diphtheria, tetanus etc.
  • Subunit vaccines – These contain part of the vaccine. Examples include: vaccine against Hepatitis B with protein subunits; the virus-like particle (VLP) vaccine against human papillomavirus (HPV) and the hemagglutinin and neuraminidase subunits of the influenza virus.
  • Conjugate vaccines – These contain the polysaccharide outer coats with proteins or toxins. Examples include ''Haemophilus influenzae'' type B vaccine.

Vaccine Schedule

Vaccines need to be administered at particular designated time in life to prevent the infection. For maximum effectiveness children are recommended to receive vaccinations as soon as their immune systems are sufficiently developed to respond to the components of the vaccines. Additional booster shots are required to teach the body fully to deal with the infection later in life. This requirement has led to the development of complex vaccination schedules.

How are vaccine schedules decided upon?

The schedules are decided upon by organizations. These schedules are decided upon based on the needs of the community, the local prevalence of the disease etc.

In the United States, the Advisory Committee on Immunization Practices recommends schedule additions for the Centers for Disease Control and Prevention. The ACIP recommends vaccinations of all children against:

  • hepatitis A
  • hepatitis B
  • polio
  • mumps
  • measles
  • rubella
  • diphtheria
  • pertussis
  • tetanus
  • HiB
  • chickenpox
  • Rotavirus
  • Influenza
  • meningococcal disease
  • pneumonia

The schedules also recommend vaccines and boosters for older children, adolescents, adults, pregnant women, travellers and the elderly.

Combined vaccines

There may be up to 24 injections by the age of 2. Too many injections and complex schedules mean less compliance among general population. One way to deal with this is introduction of combined vaccines like MMR against measles, mumps and rubella.

Vaccines and prophylactic measures for travellers

Those who travel to countries where certain infections are prevalent are also at a risk of acquiring infections. CDC divides vaccines for travel into three categories: routine, recommended, and required.

Some routine vaccines are important for travel. These include the routine vaccines for children and adults (figures above). These vaccines are recommended to protect travellers from illnesses present in other parts of the world and to prevent the importation of infectious diseases across international borders.

Some specific travel related vaccines include yellow fever vaccination for travel to certain countries in sub-Saharan Africa and tropical South America. Meningococcal vaccination is required by the government of Saudi Arabia for annual travel during the Hajj etc.



Vaccine Effectiveness

Vaccines help the body remember a microorganism to be able to fight it off when the need comes. However, vaccines are not fool proof and do not guarantee complete protection from a disease.

This could be due to various reasons. Sometimes this is because the host's immune system simply doesn't respond adequately or at all. This could be in diseased persons with lowered immunity e.g. in diabetics, those on steroids or other immunity suppressing drugs or those with HIV infection.

The reason for non-development of immunity to a disease could also be because the host's immune system does not have a B cell capable of generating antibodies against the antigen or microbe or the immune system may not be strong enough to fight off the infection.

Efficacy of a vaccine

The efficacy of a vaccine is different from its effectiveness. It is dependent on several factors such as:

  • adherence to vaccine schedules
  • the disease – some diseases have vaccines against them that perform better than for other diseases
  • the strain of vaccine – some vaccine strains are more effective than others
  • non-responders – some individuals do not respond to certain vaccines at all. The cause may lie in the race, ethnicity or genetics of the individual

The mathematical deduction of protective vaccine efficacy is nearly 100 years old, having been proposed by Greenwood and Yule in 1915 for inactivated whole cell cholera and typhoid vaccines.

Measuring vaccine efficacy

Vaccine efficacy is best measured by double-blind, clinical trials. These explore the “best case scenarios” of vaccine protectiveness under controlled conditions and are commonly required before a new vaccine is licensed by the Food and Drug Administration and other global regulatory authorities.

The outcome of efficacy is measured by parameters like - proportionate reduction in disease attack rate (AR) between the unvaccinated (ARU) and vaccinated (ARV) individuals in the clinical trial. This can give the relative risk of getting the disease (RR) of the disease after use of the vaccine.

Efficacy = (ARU-ARV)/ARU X 100


Efficacy = (1-RR) X 100

Advantages and disadvantages of vaccine efficacy

While advantages of knowing the vaccine efficacy means it has been tried in strict clinical conditions, the disadvantages that it has not been tried on larger general populations. Vaccine efficacy studies can measure outcomes beyond disease attack rates, including hospitalizations, medical visits, and costs.

Vaccine effectiveness and “field efficacy”

Vaccine effectiveness was initially termed “field efficacy”. Essentially, vaccine effectiveness is a “real world” view of how a vaccine reduces disease in a population. This vaccine may already have been proved to be efficacious in clinical trials. This measure can assess the net balance of benefits and adverse effects of a vaccination program rather than the vaccine alone in field conditions.

Vaccine effectiveness

Vaccine effectiveness is proportional to vaccine potency or vaccine efficacy but is primarily affected by how well target groups in the population are immunized, difficulties in storing, administering, cost, accessibility, availability, stability and manufacturing of the vaccine.

Effectiveness is expressed as a rate difference. It uses odds ratio (OR) for developing infection despite vaccination and can be derived as:

Effectiveness = (1-OR) X 100.

Considerations for effectiveness of a vaccine

  • effect of the disease on the local prevalence and incidence of the disease it is targeted against. This needs to be considered over medium and long term as well
  • continued surveillance for the relevant disease following introduction of a new vaccine
  • maintenance of high immunization rates, even when a disease has become rare
  • maintenance of availability, adequate stability, low cost and wide coverage of the vaccine