A vaccine is a medical preparation given to provide immunity from a disease. Vaccines use a variety of different substances, ranging from dead microorganisms to genetically engineered antigens, to defend the body against potentially harmful microorganisms. Effective vaccines change the immune system by promoting the development of antibodies that can quickly and effectively attack a disease-causing microorganism when it enters the body, preventing disease development.

The development of vaccines against diseases ranging from polio and smallpox to tetanus and measles is considered one of the great accomplishments of medical science. Contemporary researchers are currently attempting to develop new vaccinations against such diseases as COVID–19, HIV/AIDS, malaria, influenza, and other diseases.

Physicians have long observed that individuals who were exposed to an infectious disease and survived were somehow protected against that disease in the future. Prior to the invention of vaccines, however, infectious diseases were common and devastating.

Vaccines work using antibodies, also known as immunoglobulin, which develop when the immune system fights off a disease. Researchers at the Centers for Disease Control and Prevention (CDC) help decide what goes into the flu vaccine each year. (CDC/Robert Denty) (CDC/Robert Denty)

Vaccines operate by stimulating the immune system to produce components such as antibodies that will help fight the actual infection. Depending on the vaccine, the formulation may use an intact version of the disease-causing virus (albeit weakened in some way so that it is not able to cause the disease, as was used in the Sabin oral polio vaccine) or a fragment of a protein from the virus or bacteria that is critical to the establishment or the progression of the infection. An example of the latter approach is the use of a protein involved in the binding of the virus or bacteria to host cells. The antibodies produced can block the binding, and so hamper the development of an infection. Another example is the vaccine for Streptococcus pneumoniae, which uses bacterial polysaccharides (carbohydrates found in bacteria that contain large numbers of monosaccharides, a simple sugar). Streptococcus-related diseases often involve the coating of the bacteria in a sugary capsule composed of the saccharides; thus, antibodies against this component can be a useful means of combating the infection.

Effective vaccines have limited many life-threatening infectious diseases. In the United States, children starting kindergarten in most states are required to be immunized against polio, diphtheria, tetanus, and pertussis. Vaccines also required are measles, mumps, rubella, varicella (chickenpox), meningococcal disease, pneumococcal disease, Haemophilus influenzae type b (HiB), and hepatitis B. Additional vaccines, including rotavirus, are recommended. Other vaccinations are used only for populations at risk, or when individuals are exposed to disease. These are also given when exposure to a disease is likely to occur due to travel to an area where the disease is common. Examples of these include yellow fever, typhoid, rabies, Japanese encephalitis, and cholera.

The influenza virus is one of the more problematic diseases because the viruses constantly change, making development of vaccines difficult. Scientists grapple with Page 4630  |  Top of Articlepredicting what particular influenza strain will predominate in a given year. When the prediction is accurate, the vaccine is effective. When the prediction is inaccurate, the vaccine is often of little help.

Most vaccines are administered starting at infancy and continue through adulthood, although some vaccines may be necessary only when traveling to areas affected by certain diseases. (didesign021/Shutterstock) (didesign021/Shutterstock)

The classical methods of vaccine preparation vary in safety and efficiency. In general, vaccines that use live bacterial or viral products are extremely effective when they work, but carry a greater risk of causing disease. This is most threatening to individuals whose immune systems are weakened, such as individuals with leukemia. Children with leukemia are advised not to take the attenuated (weakened virus) oral polio vaccine, for example, because they are at greater risk of developing the disease. The polio vaccine is available in both an oral live attenuated version and an injectable inactivated vaccine (IPV). The oral vaccine is easier to administer and store, especially in developing countries, but it has resulted in cases of vaccine-derived polio among unvaccinated children, who come into contact with water contaminated with the stool of vaccinated children. World health officials, therefore, plan to push the efforts to eradicate polio by eventually phasing out the oral vaccine in favor of IPV. Vaccines that do not include a live virus or bacteria tend to be safer, but their protection may not be as long lasting.

The classical types of vaccines are all limited in their dependence on biological products, which often must be kept cold, may have a limited life, and can be difficult to produce. The development of recombinant vaccines—those using chromosomal parts (or deoxyribonucleic acid, commonly called DNA) from a different organism—has generated hope for a new generation of man-made vaccines. The hepatitis B vaccine, one of the first recombinant vaccines to be approved for human use, is made using recombinant yeast cells genetically engineered to include the gene coding for the hepatitis B antigen. Because the vaccine contains the antigen, it is capable of stimulating antibody production against hepatitis B without the risk that live hepatitis B vaccine carries by introducing the virus into the blood stream.

As medical knowledge has increased—particularly in the field of DNA vaccines—researchers have set their sights on possible new vaccines for cancer, melanoma, AIDS, influenza, and numerous others. Many improved vaccines have been approved, including several genetically engineered (recombinant) types that were first developed during an experiment in 1990. These recombinant vaccines involve the use of so-called “naked DNA.” Microscopic portions of DNA from a virus are injected into the patient. The patient's own cells then adopt that DNA, which is then duplicated when the cell divides, becoming part of each new cell. Researchers have reported success using this method in laboratory trials against influenza and malaria. These DNA vaccines work from inside the cell, not just from the cell's surface, as other vaccines do, allowing a stronger cell-mediated fight against the disease. Also, because the influenza virus constantly changes its surface proteins, the immune system or vaccines cannot change quickly enough to fight each new strain. However, DNA vaccines work on a core protein, which researchers believe should not be affected by these surface changes.

Since the emergence of HIV and AIDS in the early 1980s, a worldwide search against the disease has resulted in clinical trials for more than 25 experimental vaccines. These range from whole-inactivated viruses to genetically engineered types. Some have focused on a therapeutic approach to help infected individuals to fend off further illness by stimulating components of the immune system; others have genetically engineered a protein on the surface of HIV to prompt immune response against the virus; and yet others attempted to protect uninfected individuals. The challenges in developing a protective vaccine include the fact that HIV appears to have multiple viral strains and mutates quickly. As of 2020, no AIDS vaccine had been approved for use.

A vaccine against malaria, the first anti-parasitic vaccine ever developed for humans, received approval from the European Medicines Agency in July 2015. After three decades of development and trials in seven sub-Saharan African countries, the Mosquirix vaccine is designed specifically to prevent malaria infection in children. Although it provides only partial protection (up to 30 percent of malaria infections were prevented in trials), Mosquirix could play a significant role in malaria prevention strategies in countries where rates of the disease are high. The vaccine also presents challenges in delivery; it requires three monthly injections given later than the Page 4631  |  Top of Articlestandard course of infant immunizations, plus a booster. Nevertheless, scientists consider Mosquirix as a foundation upon which to build more effective malaria vaccines. The British pharmaceutical company GlaxoSmithKline developed the vaccine and also pledged to receive no profit from it. In 2019, Mosquirix was introduced into Ghana, Kenya, and Malawi—a type of test run monitored by the World Health Organization (WHO). If the test runs positive, a larger distribution will commence in 2021.

In one of the largest and fastest mass vaccination campaigns carried out in history, more than seven million people in Kinshasa, capital city of the Democratic Republic of the Congo, were vaccinated against yellow fever during a two-week period in August 2016. Almost four million more people living in Kinshasa province and remote areas along the Congo-Angola border were also targeted for vaccination, as an ongoing yellow fever outbreak killed more than 400 people in the region. The emergency vaccination campaign used a vaccine that was diluted by a factor of five in order to vaccinate as many people as possible with existing supplies and halt the epidemic; this effort provides a potential model for future emergency vaccine responses. The diluted vaccine provided sufficient immunity to protect the individual from yellow fever infection during the epidemic, but had an estimated effective duration of about one year, as opposed to ten years or more for the traditional-strength vaccine.

The reluctance of some parents to vaccinate their children due to potential side effects has lowered vaccination use. Parents in the United States and several European countries have balked at vaccinating their children with the pertussis vaccine due to the development of neurological complications in a small number of children given the vaccine. Nevertheless, the risk of infection is far greater than the risk of vaccine-related complications. Because of incomplete immunization, whooping cough (pertussis) remains in the United States, with more than 15,600 cases identified in 2019.

In 2012, a California Department of Public Health study found that the number of unvaccinated kindergarten students who received personal beliefs vaccine exemptions rose 25 percent between 2008 and 2010. California law at that time allowed for parents to opt out of vaccinating their school-aged children if vaccination is contrary to their personal beliefs. The study further identified schools with high percentages of unvaccinated students, with health officials warning that the vaccination rate in some may be below herd immunity levels. Herd immunity offers some protection for unvaccinated individuals by disrupting avenues of disease transmission and reducing the occurrence or severity of contagious disease epidemics. In 2015, the law was knocked down, but the rate of unvaccinated students continues to climb as parents are using a medical exemption clause to keep their children unvaccinated.

In 2015, Norway experienced a jump in reported cases of mumps (kusma in Norwegian) in college students. Public health officials attributed the spike in cases to parental resistance to Norway's Childhood Immunization Program, and incomplete adherence to recommended vaccination schedules. The WHO declared resistance to vaccines and the anti-vaccine movement among the top threats to global health in 2019.

Resurgent epidemics occur among populations where vaccination rates fall below recommended levels. Breakout infections are very rare, but can occur when infections strike well-vaccinated populations. The vast majority of people are completely protected by vaccines specific for the intended disease and pathogens, but in general, there is always a very small number of people for whom vaccination confers less than complete protection due to variations in individual genetics, medical history, vaccine batch, adherence to recommended vaccine schedule, time since vaccination, and other factors.