Active Immunization Elicits Long Term Protection

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Whereas the aim of passive immunization is transient protection or alleviation of an existing condition, the goal of active immunization is to elicit protective immunity and immunologic memory. When active immunization is successful, a subsequent exposure to the pathogenic agent elicits a heightened immune response that successfully eliminates the pathogen or prevents disease mediated by its products. Active immunization can be achieved by natural infection with a microorganism, or it can be acquired artificially by administration of a vaccine (see Table 18-1). In active immunization, as the name implies, the immune system plays an active role—proliferation of antigen-reactive T and B cells results in the formation of memory cells. Active immunization with various types of vaccines has played an important role in the reduction of deaths from infectious diseases, especially among children.

Vaccination of children is begun at about 2 months of age. The recommended program of childhood immunizations in this country, updated in 2002 by the American Academy of Pediatrics, is outlined in Table 18-3. The program includes the following vaccines:

Hepatitis B vaccine

■ Diphtheria-pertussis (acellular)-tetanus (DPaT) combined vaccine

■ Inactivated (Salk) polio vaccine (IPV); the oral (Sabin) vaccine is no longer recommended for use in the United States

■ Measles-mumps-rubella (MMR) combined vaccine

■ Haemophilus influenzae (Hib) vaccine

■ Varicella zoster (Var) vaccine for chickenpox

■ Pneumococcal conjugate vaccine (PCV); a new addition to the list.

In addition, hepatitis A vaccine at 18 months and influenza vaccines after 6 months are recommended for infants in high-risk populations.

The introduction and spreading use of various vaccines for childhood immunization has led to a dramatic decrease in the incidence of common childhood diseases in the United States (Figure 18-1). The comparisons of disease incidence in 1999 to that reported in the peak years show dramatic drops and, in one case, complete elimination of the disease in the United States. As long as widespread, effective immunization programs are maintained, the incidence of these childhood diseases should remain low. However, the occurrence of side reactions to a vaccine may cause a drop in its use, which can lead to re-emergence of that disease. For example, the side effects from the pertussis attenuated bacterial vaccine included seizures, encephalitis, brain damage, and even death. Decreased usage of the vaccine led to an increase in the inci-

TABLE 18-3 Recommended childhood immunization schedule in the United States, 2002

Vaccine* Birth 1 mo 2 mos 4 mos 6 mos 12 mos 15 mos 18 mos 4-6 yrs

Hepatitis Br +

pertussis*

pertussis*

H. influenzae, type b

+

+

1 |

+

Inactivated polio§

+

+

+

1

Pneumococcal conjugate

+

+

1 |

+

Measles, mumps, rubella + +

Varicella# +

Measles, mumps, rubella + +

Varicella# +

*This schedule indicates the recommended ages for routine administration of currently licensed childhood vaccines. Bars indicate ranges of recommended ages. Any dose not given at the recommended age should be given as a "catch-up" immunization at any subsequent visit when indicated and feasible.

^Different schedules exist depending upon the HBsAg status of the mother. A first vaccination after the first month is recommended only if the mother is HBsAg negative.

*DTaP (diphtheria and tetanus toxoids and acellular pertussis vaccine) is the preferred vaccine for all doses in the immunization series. Td (tetanus and diphtheria toxoids) is recommended at 11-12 years of age if at least 5 years have elapsed since the last dose.

§Only inactivated poliovirus (IPV) vaccine is now recommended for use in the United States. However, OPV remains the vaccine of choice for mass immunization campaigns to control outbreaks due to wild poliovirus.

#Varicella (Var) vaccine is recommended at any visit on or after the first birthday for susceptible children, i.e., those who lack a reliable history of chickenpox (as judged by a health-care provider) and who have not been immunized. Susceptible persons 13 years of age or older should receive 2 doses, given at least 4 weeks apart.

SOURCE: Adapted from the ECBT Web site (see references); approved by the American Academy of Pediatrics.

Rubella 1969

Paralytic polio 1952

Pertussis 1934

Mumps 1968

Measles 1941

Diphtheria 1921

0 cases

Number of reported cases

FIGURE 18-1

Reported annual number of cases of rubella (German measles), polio, pertussis (whooping cough), mumps, measles, and diphtheria in the United States in the peak year for which data are available (orange) compared with the number of cases of each disease in 1999 (green). Currently, vaccines are available for each of these diseases, and vaccination is recommended for all children in the United States. [Data from Centers for Disease Control.]

dence of whooping cough, with 7405 cases in 1998. The recent development of an acellular pertussis vaccine that is as effective as the older vaccine, but with none of the side effects, is expected to reverse this trend.

As indicated in Table 18-3, children typically require multiple boosters (repeated inoculations) at appropriately timed intervals to achieve effective immunity. In the first months of life, the reason for this may be persistence of circulating maternal antibodies in the young infant. For example, passively acquired maternal antibodies bind to epitopes on the DPT vaccine and block adequate activation of the immune system; therefore, this vaccine must be given several times after the maternal antibody has been cleared from an infant's circulation in order to achieve adequate immunity. Passively acquired maternal antibody also interferes with the effectiveness of the measles vaccine; for this reason, the MMR vaccine is not given before 12-15 months of age. In Third World countries, however, the measles vaccine is administered at 9 months, even though maternal antibodies are still present, because 30%-50% of young children in these countries contract the disease before 15 months of age.

Multiple immunizations with the polio vaccine are required to ensure that an adequate immune response is generated to each of the three strains of poliovirus that make up the vaccine.

Recommendations for vaccination of adults depend on the risk group. Vaccines for meningitis, pneumonia, and influenza are often given to groups living in close quarters (e.g., military recruits) or to individuals with reduced immunity (e.g., the elderly). Depending on their destination, international travelers are also routinely immunized against such endemic diseases as cholera, yellow fever, plague, typhoid, hepatitis, meningitis, typhus, and polio. Immunization against the deadly disease anthrax had been reserved for workers coming into close contact with infected animals or products from them. Recently, however, suspected use of anthrax spores by terrorists or in biological warfare has widened use of the vaccine to military personnel and civilians in areas at risk of attack with this deadly agent.

Vaccination is not 100% effective. With any vaccine, a small percentage of recipients will respond poorly and therefore will not be adequately protected. This is not a serious problem if the majority of the population is immune to an infectious agent. In this case, the chance of a susceptible individual contacting an infected individual is so low that the susceptible one is not likely to become infected. This phenomenon is known as herd immunity. The appearance of measles epidemics among college students and unvaccinated preschool-age children in the United States during the mid-to late 1980s resulted partly from an overall decrease in vaccinations, which had lowered the herd immunity of the population (Figure 18-2). Among preschool-age children, 88% of those who developed measles were unvaccinated. Most of the college students who contracted measles had been vaccinated as children, but only once; the failure of the single vaccination to protect them may have resulted from the presence of passively acquired maternal antibodies that reduced their overall response to the vaccine. The increase in the incidence of measles prompted the recommendation that children receive two immunizations with the combined measles-mumps-rubella vaccine, one at 12-15 months of age and the second at 4-6 years.

The Centers for Disease Control (CDC) has called attention to the decline in vaccination rates and herd immunity among American children. For example, a 1995 publication reported that in California nearly one-third of all infants are unvaccinated and about half of all children under the age of 2 are behind schedule on their vaccinations. Such a decrease in herd immunity portends serious consequences, as illustrated by recent events in the newly independent states of the former Soviet Union. By the mid-1990s, a diphtheria epidemic was raging in many regions of these new countries, linked to a decrease in herd immunity resulting from decreased vaccination rates after the breakup of the Soviet Union. This epidemic, which led to over 157,000 cases of diptheria and 5000 deaths, is now controlled by mass immunization programs.

400 300 200

400 300 200

1950 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84

Year

86 88

1950 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84

Year

86 88

FIGURE 18-2

Introduction of the measles vaccine in 1962 led to a dramatic decrease in the annual incidence of this disease in the United States. Occasional outbreaks of measles in the 1980s (inset)

occurred mainly among unvaccinated young children and among college students; most of the latter had been vaccinated, but only once, when they were young. [Data from Centers for Disease Control.]

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  • Osvaldo
    Why active immunization elicts long term protection?
    2 years ago

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