Types of Vaccines

Vaccines can be made with live organisms or with organisms killed by heat or chemicals. If live organisms are used, they must be nonvirulent for humans, such as the cowpox virus used for smallpox immunization, or they must be treated in the laboratory to weaken them as human pathogens. An organism weakened for use in vaccines is described as attenuated. In some cases, just an antigenic component of the pathogen is used as a vaccine. Another type of vaccine is made from the toxin produced by a disease organism. The toxin is altered with heat or chemicals to reduce its harmfulness, but it can still function as an antigen to induce immunity. Such an altered toxin is called a toxoid. The newest types of vaccines are produced from antigenic components of pathogens or by genetic engineering. By techniques of recombinant DNA, the genes for specific disease antigens are inserted into the genetic material of harmless organisms. The antigens produced by these organisms are extracted and purified and used for immunization. The hepatitis B vaccine is produced in this manner.

Boosters In many cases, an active immunity acquired by artificial (or even natural) means does not last a lifetime. Circulating antibodies can decline with time. To help maintain a high titer (level) of antibodies in the blood, repeated inoculations, called booster shots, are administered at intervals. The number of booster injections recommended varies with the disease and with an individual’s environment or range of exposure. On occasion, epidemics in high schools or colleges may prompt recommendations for specific boosters. The number and timing of doses varies with the different vaccines.

Examples of Bacterial Vaccines Children are routinely immunized with vaccines against bacteria or their toxins. Because of whooping cough’s seriousness in young infants, early inoculation with whooping cough, or pertussis, vaccine, is recommended. A new form of the vaccine containing pertussis toxoid causes fewer adverse reactions than older types that contained heat-killed organisms. This acellular (aP) vaccine usually is given in a mixture with diphtheria toxoid and tetanus toxoid. The combination, referred to as DTaP, may be given as early as the second month of life and should be followed by additional injections at 4, 6, and 15 months and again when the child enters day care, a school, or any other environment in which he or she might be exposed to one of these contagious diseases. Diphtheria and tetanus toxoid (Td) is given again at 11 to 12 years of age. A tetanus booster is given when there is a disease risk and the last booster was administered more than 10 years prior to exposure. Routine inoculation against Haemophilus influenzae type B (Hib) has nearly eliminated the life-threatening meningitis caused by this organism among preschool children. Hib also causes pneumonia and recurrent ear infections in young children. Depending on the type used, the vaccine is given in either two doses or three doses beginning at 2 months of age. Pneumococcal vaccine (PCV) protects against infection with pneumococcus, an organism that can cause pneumonia and meningitis. Four doses are given between the ages of 2 and 15 months.

Examples of Viral Vaccines Intensive research on viruses has resulted in the development of vaccines for an increasing number of viral diseases. The medical community has achieved spectacular results in eliminating poliomyelitis by the use of vaccines. The first of these was an inactivated polio vaccine (IVP) developed by Dr. Jonas Salk and made with killed poliovirus. A more convenient oral vaccine (OPV), made with live attenuated virus, was then developed by Dr. Albert Sabin. Both vaccine types are presently used in worldwide immunization programs, but IPV is preferred for routine childhood immunizations. A series of three doses is given between 2 and 18 months, and a fourth dose is given before entry into school. MMR, made with live attenuated viruses, protects against measles (rubeola), mumps, and rubella (German measles). Rubella is a very mild disease, but it causes birth defects in a developing fetus. A first dose of MMR is given at 15 months and a second between 4 and 6 years of age.
Infants are now routinely immunized against hepatitis B, receiving the first of three shots just after birth and two more before the age of 18 months. The vaccine is also recommended for people at high risk of hepatitis B infection, including healthcare workers, people on kidney dialysis, people receiving blood clotting factors, injecting drug users, and those with multiple sexual partners. A vaccine against hepatitis A virus is recommended for travelers and others at high risk for infection. A vaccine against chicken pox (varicella) has been available since 1995. Children who have not had the disease by 1 year of age should be vaccinated. Although chicken pox is usually a mild disease, it can cause encephalitis, and infection in a pregnant woman can cause congenital malformation of the fetus. Because varicella is the same virus that causes shingles, vaccination may prevent this late-life sequel. A number of vaccines have been developed against influenza, which is caused by a variety of different viral strains. Laboratories produce a new vaccine each year to combat what they expect will be the most common strains in the population. The elderly, the debilitated, and children with certain risk factors, including asthma, heart disease, sickle cell disease, HIV infection, and diabetes, should be immunized yearly against influenza. The rabies vaccine is an exception to the rule that a vaccine should be given before invasion by a disease organism. Rabies is a viral disease transmitted by the bite of wild animals such as raccoons, bats, foxes, and skunks. Mandatory vaccination of domestic animals has practically eliminated this source of rabies in some countries, including the United States, but worldwide, a variety of wild and domestic animals are host to the virus.
There is no cure for rabies; it is fatal in nearly all cases. The disease develops so slowly, however, that affected people vaccinated after transmission of the organism still have time to develop an active immunity. The vaccine may be given preventively to people who work with animals.
Passive Immunization It takes several weeks to produce a naturally acquired active immunity and even longer to produce an artificial active immunity through the administration of a vaccine. Therefore, a person who receives a large dose of virulent organisms and has no established immunity to them is in great danger. To prevent illness, the person must quickly receive counteracting antibodies from an outside source. This is accomplished through the administration of an immune serum, or antiserum. The “ready-made” serum gives short-lived but effective protection against the invaders in the form of an artificially acquired passive immunity. Immune sera are used in emergencies, that is, in situations in which there is no time to wait until an active immunity has developed.

Preparation of Antisera Immune sera often are derived from animals, mainly horses. It has been found that the horse’s tissues produce large quantities of antibodies in response to the injection of organisms or their toxins. After repeated injections, the horse is bled according to careful sterile technique; because of the animal’s size, it is possible to remove large quantities of blood without causing injury. The blood is allowed to clot, and the serum is removed and packaged in sterile containers. Injecting humans with serum derived from animals is not without its problems. The foreign proteins in animal sera may cause an often serious sensitivity reaction, called serum sickness. To avoid this problem, human antibody in the form of gamma globulin may be used.

Examples of Antisera Some immune sera contain antibodies, known as antitoxins, that neutralize toxins but have no effect on the toxic organisms themselves. Certain antibodies act directly on pathogens, engulfing and destroying them or preventing their continued reproduction. Some antisera are obtained from animal sources, others from human sources. Examples of immune sera are:
* Diphtheria antitoxin, obtained from immunized horses.
* Tetanus immune globulin, effective in preventing lockjaw (tetanus), which is often a complication of neglected wounds. Because tetanus immune globulin is of human origin, it carries less risk of adverse reactions than do sera obtained from horses.
* Immune globulin (human) is given to people exposed to hepatitis A, measles, polio, or chickenpox. It is also given on a regular basis to people with congenital (present at birth) immune deficiencies.
* Hepatitis B immune globulin, used after hepatitis B exposure, is given principally to infants born to mothers who have hepatitis.
* The immune globulin Rho(D) (trade name RhoGAM), a concentrated human antibody given to prevent an Rhnegative mother from forming Rh antibodies. It is given during pregnancy if maternal antibodies develop and after the birth of an Rh-positive infant (or even after a miscarriage of a presumably Rh-positive fetus). It is also given when Rh transfusion incompatibilities occur.
* Anti-snake bite sera, or antivenins are used to combat the effects of certain poisonous snake bites.
* Botulism antitoxin, an antiserum from horses offers the best hope for botulism victims, although only if given early.
* Rabies antiserum, from humans or horses, is used with the vaccine to treat victims of rabid animal bites.
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