Vaccination/Herd Immunity

Vaccination is the medical procedure in which protection is intended through the induction of immunological responses to specific agents. Vaccination is the administration of antigenic material to produce immunity to a disease. In some cases, antigens of different agents are placed together in the same vaccine, thus protecting against various diseases at the same time (e.g., viral trivalent or MMR—mumps, measles, and rubella). Some vaccines can prevent disease (e.g., measles or yellow fever vaccines), and others can mitigate the effects of infection by a pathogen (e.g., BCG vaccine or Bacillus Calmette-Guerin for tuberculosis).

Vaccination with effective antigenic materials has revolutionized the interventions in public health since Edward Jenner's discovery in 1796. Jenner tested the possibility of using a cowpox vaccine as an immunization for smallpox in humans for the first time; he is considered the father of immunology and vaccinology. Vaccination (Latin: vacca, cow) is so named because the first vaccine was derived from a virus affecting cows (cowpox virus), which provides a degree of immunity to smallpox in humans.

Despite today's safe and effective vaccines, which are widely available for many diseases—considered vaccine-preventable—many of them continue to be a public health problem because of a wide range of cultural and religious issues in many parts of the world. This is particularly true in Africa and Asia. Diseases such as poliomyelitis were eradicated from the Americas in 1991—the last indigenous case occurred in a 3-year-old boy, Luis Fermin Tenorio, from Junin in northern Peru. Polio is still a significant public health problem, however, given the virus's transmission and its morbidity and mortality in children. In 2008, 1,651 confirmed cases of polio were reported: 798 in Nigeria, 559 in India, and 117 in Pakistan, among other endemic countries.

In this context, it is very important to introduce the concept of herd immunity (or community immunity), which describes an immunological protection that occurs when the vaccination of a fraction of the population provides protection to unprotected individuals. Herd immunity theory proposes that in diseases passed from person to person, it is more difficult to maintain a chain of infection when large numbers of a population are immune. The higher the proportion of individuals who are immune, the lower the likelihood that a susceptible person will come into contact with an infected individual.

From the public health perspective, vaccinations and herd immunity are of utmost importance in the prevention of many diseases, as well as in the preservation of health, particularly in children but also in adults.

Annually, in the United States, the Advisory Committee on Immunization Practices (http://www.cdc.gov/vaccines/recs/acip), the American Academy of Pediatrics (http://www.aap.org), the American Academy of Family Physicians (http://www.aafp.org), and the Centers for Disease Control and Prevention (http://www.cdc.gov) develop the Recommended Immunization Schedules. In 2009 the following vaccines for persons aged 0 through 6 years were included in those recommendations: hepatitis B virus vaccine (HepB), to be applied at minimum age of birth; rotavirus vaccine (RV), to be applied at minimum age of 6 weeks; diphtheria and tetanus toxoids and acellular pertussis vaccine (DTaP), to be applied at minimum age of 6 weeks; Haemophilus influenzae type b conjugate vaccine (Hib), to be applied at minimum age of 6 weeks; pneumococcal vaccine, to be applied at minimum age of 6 weeks for pneumococcal conjugate vaccine (PCV) and 2 years for pneumococcal polysaccharide vaccine (PPSV); influenza vaccine, to be applied at minimum age of 6 months for trivalent inactivated influenza vaccine (TIV) and 2 years for live, attenuated influenza vaccine (LAIV); MMR, to be applied at minimum age of 12 months; varicella vaccine, to be applied at minimum age of 12 months; hepatitis A vaccine (HepA), to be applied at minimum age of 12 months; and meningococcal vaccine, to be applied at minimum age of 2 years for meningococcal conjugate vaccine (MCV) and for meningococcal polysaccharide vaccine (MPSV).

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