Immunization is a procedure routinely used to improve the body’s ability to overcome infection and protect against diseases caused by infectious agents. It works against a specific disease by training the immune system to rapidly recognize and eliminate the infectious agent that causes that disease, thus resulting in immunity. Protection can be acquired either by passive or by active immunization.

Passive immunization involves transfer of antibodies obtained from an immune donor to a nonimmune individual and results in temporary immunity. Currently, antibody-based therapies represent a form of treatment for disorders induced by venoms or toxins and for viral infections. Injections of antibody preparations derived from immunized human donors are used for the prophylaxis and treatment of tetanus, rabies, and pneumonia caused by respiratory syncytial virus (RSV), as well as infections caused by hepatitis A  virus,  hepatitis  B  virus,  and  varicella zoster virus. Monoclonal antibodies are expected to replace preparations derived from human donors. However, at this time, only one monoclonal antibody has been licensed for an infectious disease (RSV infection).

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Active immunization induces an adaptive, long-lasting immune response to a pathogen by introducing the killed or attenuated pathogens or antigens derived  from  the  pathogens  into  the  body. These nonpathogenic forms of the pathogen are the major component of a vaccine. Therefore, active immunization is routinely achieved by administering different types of vaccines. In the past 100 years, the use of vaccines (along with sanitation practices) has dramatically reduced the number of deaths caused by infectious diseases. Administration of vaccines results in the induction of a vigorous immune response, similar to the one that would be induced by natural infection. Vaccine administration may result  in  antibody  responses,  cellular  responses, or both. A major feature of effective vaccines is the ability to induce memory immunity, which allows the immune system to respond quickly and strongly to infections even several years after immunization. However, in order to maintain sustained, long-term responses, it is necessary to give multiple doses of a vaccine.

Due to the dangers associated with administering a live, infectious agent, attenuated or killed pathogens are routinely used as vaccines. Attenuated pathogens lose their ability to cause disease while maintaining their capacity to transiently grow within the immunized individual. Killed or inactivated vaccines are produced by killing pathogens with chemical or heat treatments. Certain other types of vaccines dispense with the whole organism and use just the important parts that will stimulate an immune response. Such vaccines are called subunit vaccines and consist of inactivated toxins (toxoids), capsular polysaccharides, or recombinant protein antigens.

In 1798, Edward Jenner developed the first live vaccine for smallpox from the cowpox virus. Jenner observed that a person who contracted cowpox, a much milder disease, would not get smallpox. He inoculated a farm boy with fluid from cowpox lesions of a milkmaid. Six weeks later, Jenner injected the boy with fluid from a smallpox sore. As expected, the boy did not develop the infection. Jenner’s discovery established the general principles of safe and effective vaccination and resulted in a sharp decline in the death rate from smallpox in Europe and North America. The use of improved smallpox vaccines eventually led to the eradication of this disease in 1980. However, doses of smallpox vaccine are once again being stockpiled around the world because of concerns related to bioterrorism. Jenner’s success with the smallpox vaccine set the foundation for future vaccine development. In 1881, Louis Pasteur developed the first heat-killed anthrax bacilli vaccine. He also proposed that attenuated forms of a virus could be used for immunizations against more virulent forms and developed the rabies vaccine in 1885. Jonas Salk introduced an inactivated vaccine for polio in 1955, which was composed  of  killed  poliovirus.  In  1959, Albert  Sabin developed an oral polio vaccine from live, weakened strains of the poliovirus. In 1988, the World Health Assembly resolved to eradicate polio by the year 2005. The Global Polio Eradication Initiative is now the largest public health initiative in history. Global immunization rates for the oral polio vaccine peaked in 1990 at 83%. The number of polio cases worldwide has decreased from 350,000 in 1988 to fewer than 800 cases in 2003. Three quarters of all cases globally are now linked to only a few polio “hot spots.” Before the measles vaccine was approved in 1963, measles was common in childhood, with more than 90% of infants and children infected by 12 years of age. Since 1997, there have been fewer than 150 measles cases reported annually in the United States. Measles is the only other infectious disease currently targeted for eradication.

Immunization programs in recent decades have achieved genuine success around the world. However, every  year,  nearly  3  million  individuals,  including 2 million children, die from diseases that could have been prevented by immunizations. Therefore, increasing attention is paid to childhood immunizations, a series of immunizations given to children to prevent diseases that pose a threat to them. Immunity provided to the newborn by maternal antibodies transferred through the placenta protects against many diseases. However, this immunity generally declines during the first year of life. In addition, the majority of children do not have maternal antibodies against whooping cough, polio, hepatitis B, or infections due to Hemophilus influenzae type B, all diseases that can be prevented by currently available vaccines. Thus, immunization programs are started early, with many vaccines given during the first months of life. The recommended immunizations for children and adolescents in the United States. currently include the following vaccines: HepB (hepatitis B), DTaP (diphtheria, tetanus, pertussis), Hib (Haemophilus Influenzae type b), IPV (inactivated poliovirus), MMR (measles, mumps, rubella), varicella, PCV (pneumococcal), influenza (for children with certain risk factors) and, in selected populations, hepatitis A.

Despite the successes of immunization programs in most parts of the world, immunization coverage rates haven’t reached their potential in some areas. Coverage rates are constantly used as an indicator of the health of a population and refer to the percentage of a defined population protected by immunizations against specific vaccine-preventable diseases. In areas where low coverage rates exist, not enough people get vaccinated, and subsequently the nonimmunized population remains susceptible to disease. Only when sufficient numbers of people are vaccinated is adequate protection of that population achieved. Indeed, when the number of individuals susceptible to a specific infectious disease in a population decreases because of immunization, the natural reservoir of infected individuals present in that population falls, thereby reducing the probability of transmission of infection. This phenomenon, which is called herd immunity, explains why even nonimmunized members in a population can be protected from that specific infectious disease if the majority is immunized.

The coverage rates of the DTP vaccine is a useful indicator of the performance of immunization programs in a country because it is administered on a routine basis as compared to other vaccines. In 1999, the Americas and the Caribbean, the Middle East and North Africa, and East Asia and the Pacific had coverage rates close to 90%, with some regional variations. However, even though global coverage of 72% for routine  DTP  vaccination  was  achieved,  regions  of sub-Saharan Africa and South Asia fell well below the average. Globally, among vaccine-preventable diseases, measles is responsible for most deaths in children under 5 years of age. While western and central Europe, the Americas, and the Caribbean have achieved coverage of above 90% with the measles vaccine, other regions lag far behind with coverage levels at about 50%.

Political, economic, and social instability have undermined immunization programs in different parts of the world, as seen with declining coverage rates in sub-Saharan Africa and in central and east Europe. Poor health service delivery systems, lack of cold chain equipment, and improper administration have compromised the quality and safety of vaccines in some  developing  countries.  Budgetary  constraints, low political commitment, and lack of effective disease surveillance have also limited the effectiveness of immunizations in these areas.

To  reduce  barriers  and  increase  immunization rates, federally funded public health centers in the United States are required to conduct regular assessment and feedback of vaccination rates. To maintain high vaccine coverage, the Centers for Disease Control and Prevention have developed the Clinic Assessment Software Application (CASA) to assist in measuring vaccination rates. CASA also assists in improving vaccination practices, encouraging parents to initiate the vaccination series on time, and contacting parents when children are due for or have missed vaccinations. The Advisory Committee on Immunization Practices, which periodically reviews the U.S. childhood and immunization schedule, recommends regular feedback about vaccine-delivery practices in order  to  motivate  providers  and  staff  in  personal health care services. Around the world, Global Alliance in Vaccines and Immunization is involved in efforts to improve overall public health infrastructures that can monitor disease patterns. These organizations also explore strategies to increase overall financing for immunizations and raise the visibility of immunizations, especially among vulnerable populations. Mass immunization campaigns have historically played an important role in starting new immunization programs in developing countries and in areas with political instability, wars, and epidemics. Therefore, several approaches are being undertaken in the United States and around the world to maintain the remarkable progress of the various immunization programs, which, in general, continue to represent an impressive public health success with benefits extending to future generations.


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