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Vaccine-Preventable Diseases: An Overview

Global statistics from eight vaccine-preventable diseases

Source: WHO World Health Report 1999, and WHO Department of Vaccines and Biologicals

Summary of Major Vaccine-Preventable Diseases

Polio

• Polio is caused by a viral infection (poliovirus) that invades the nervous system and can cause paralysis in a matter of hours. One in 200 infections leads to paralysis (usually the legs) and its effects are irreversible. Between 5 and 10 percent of patients die when their breathing muscles are paralysed.
  
• People catch the virus through direct person-to-person contact, through contact with infected secretions from the nose or mouth or by contact with infected feces. The virus enters through the mouth and nose, multiplies in the throat and intestinal tract, and then is absorbed and spread through the blood and to the spinal cord and brain.
  
• The virus begins to cause symptoms between 5 to 35 days (average 7 to 14 days) after infection. According to the World Health Organization, it can strike at any age, but over 50 percent of all cases involve children under three. There is no cure for polio; immunization is the only way to prevent the disease.
  

Diphtheria

• Diphtheria is an infectious disease that spreads from person to person through respiratory droplets from the throat via coughing and sneezing. The disease normally breaks out 2 to 5 days after infection. Two forms of diphtheria bacteria occur: one produces toxin and the other does not. Damage is done by the toxin.
  
• The disease usually affects the tonsils, pharynx, larynx and occasionally the skin. Symptoms range from a moderately sore throat to toxic life-threatening diphtheria of the larynx or of the lower and upper respiratory tracts. This occurs because the disease results in the formation of a membrane in the throat that may get so big it completely blocks the larynx, causing death by suffocation.
  
• Other serious complications of diphtheria involve diphtheric-myocarditis (toxic damage to heart muscles) and neuritis (toxic damage to peripheral nerves) and can be fatal - about 1 person in 10 (10 percent) still dies in spite of treatment. Untreated patients are infectious for up to four weeks.
  
• Unless immunized, children and adults may be infected repeatedly with the disease. The most effective method of epidemic control is mass immunization of the entire population. The most effective method of prevention is immunization in early childhood, often followed by booster doses at school entry.
  

Pertussis (whooping cough)

• Pertussis (whooping cough) is a highly contagious bacterial disease involving the respiratory tract and caused by infection with Bordetella pertussis bacteria. It is transmitted through direct contact with discharges from respiratory mucous membranes of infected persons carried in the air. The bacteria invade the nose and throat, the trachea, and the bronchial tubes of the lungs, causing symptoms in about a week.
  
• The typical illness lasts 6 to 12 weeks, starting with symptoms similar to the common cold and progressing to spasms of coughing after 10 to 12 days.
  
• Worldwide, B. pertussis causes some 45 million cases of pertussis, 90 percent of which occur in developing countries. Although pertussis may occur at any age, most serious cases (including brain damage) and the majority of fatalities are observed in early infancy, a time when pertussis is most severe. Vaccines are the most rational approach to pertussis control - if children are not immunized, they have an 80 percent chance of contracting whooping cough before the age of five.
  

Measles

• Measles is a highly contagious viral illness characterized by a fever, cough and spreading rash. For children, it is the most contagious of the child killers. Caused by the paramyxovirus, it is spread by droplets from the nose, mouth or throat of an infected person.
• The incubation period usually lasts 10 days (with a range from 7 to 18 days) from exposure to the onset of fever. The disease is characterised by prodromal fever, conjunctivitis, coryza, cough and the presence of Koplik spots (reddish spots with a white centre) on the buccal mucosa. A characteristic red rash appears on the third to seventh day beginning on the face, becoming generalised and lasting 4 to 7 days.
• The frequency of complications varies in different parts of the world. In industrialised countries, complications occur in around 10-15 percent of cases and include diarrhoea, otitis media, pneumonia, croup and typically, encephalitis. In the developing countries at least three-quarters of cases can be expected to have at least one complication and some have multiple systems involvement. The case fatality rates in developing countries are normally estimated to be 3-5 percent, but may reach 10-30 percent in some situations. The three major causes contributing to the high case-fatality rate are pneumonia, diarrhoea and croup.
• Measles can also lead to life-long disabilities, including blindness, brain damage and deafness. Half of the childhood corneal blindness in developing countries is attributable to vitamin A deficiency, and half to measles infection. There is no specific treatment of measles but case-fatality rates can be reduced by effective case management, including the use of vitamin A supplements. Immunization is the only way to prevent the disease.

Neonatal Tetanus

• Tetanus is caused by naturally occurring bacteria that enter a body through open wounds. Sometimes called "lockjaw," it is caused by a toxin produced by bacteria called Clostridium tetani, which is found in soil, stool and manure, and on anything lying on the ground. It causes a generalized increased rigidity of skeletal muscles, causing spasms, stiffness and arching of the spine. Ultimately breathing becomes more difficult, spasms occur more frequently, and in 70 to 100 percent of the cases, mothers and infants will die.
  
• Neonatal tetanus (NT) - which affects newborn babies - generally occurs during the first few days of life and when a woman delivers her baby in unsanitary conditions, often when a delivery occurs at home with no skilled birth attendant present.
• In the absence of immunization, transmission occurs when bacteria comes in contact with broken skin, such as that resulting from umbilical wounds and circumcisions. Poverty, poor hygiene, and limited access to health services amplify the risk for disease transmission during childbirth.
  
• Neonatal tetanus is the second leading cause of death from vaccine-preventable diseases among children worldwide. Maternal tetanus - affecting the mother - also occurs as a result of poor hygienic practice at the time of delivery or through gynecological complications. Immunizing women of child-bearing age and women who are pregnant is an effective method for preventing both neo-natal and maternal tetanus.
  

Tuberculosis

• Tuberculosis (TB) is a disease caused by Mycobacterium tuberculosis. The initial infection may go unnoticed and remain latent/dormant without symptoms but others may still be infected. Every second, someone in the world is newly infected with TB, and 1.5 million die annually from the disease.
  
• Although the disease is most common in adults, it is usually more serious in infants, children and adolescents. The disease can be reactivated after years of dormancy, and is spread by prolonged close contact with an infected individual through airborne droplets from a cough or sneeze by someone who is infected with the organism.
  
• BCG vaccine (Bacillus Calmette-Guerin) was developed in 1921. Early immunization is only effective in preventing the most dangerous forms of TB (miliary forms and meningitis) which occurs primarily in children. The vaccine is given at or soon after birth, and coverage is the highest of all the Expanded Programme on Immunization (EPI) vaccines - over 85 percent globally. It has also been shown that the vaccine provides cross-protection against leprosy.
  

Haemophilus Influenzae Type b (Hib)

• Haemophilus influenzae type b (Hib) is a bacterium that can colonise the human nasopharynx. It is spread by droplets through coughs or sneezing and is often exacerbated in overcrowded living conditions. It is a leading cause of pneumonia and meningitis, mostly in young children.
  
• Pneumonia is the most common manifestation of Hib in developing countries, with an estimated 3 million cases annually. The most serious manifestation of Hib is meningitis, with a case-fatality rate of 3 percent to 5 percent in industrialized nations, and up to 30 percent in developing countries. Meningitis is marked by permanent neurological defects in 20 percent to 35 percent of survivors.
  
• Children less than 1 year of age are at highest risk of the disease, while the disease is rarely seen in children more than 5 years old. The failure to breast-feed, household crowding, and day-care attendance are risk factors for infection. Rapid diagnosis and treatment with antibiotics are essential for optimal outcome in meningitis. Immunization is one of the most cost-effective measures against this disease.
  

Hepatitis B

• Hepatitis B virus is spread from infected mothers to their infants at birth, between young children, through contaminated blood and unsafe injections and sex. Hepatitis B is the most serious type of viral hepatitis and is also the only type causing chronic disease for which a vaccine is available.
  
• Although most people are infected in infancy or childhood, most deaths from hepatitis B occur in adulthood, as a result of cirrhosis and/or liver cancer, both of which are strongly associated with the virus. Hepatitis means inflammation of the liver, and symptoms include yellowing of the skin and eyes (jaundice), dark urine, extreme fatigue, nausea and vomiting, and abdominal pain.
  
• An estimated 350 million people are chronic carriers of the virus. In the developing world (sub-Saharan Africa, most of Asia, and the Pacific) 8 percent to 15 percent of people in the general population become chronic carriers. Infection is less common in Western Europe and North America, where less than 1 percent are chronic carriers. Immunization is the only effective method to prevent the disease in infants.
  

Yellow Fever

• Yellow fever is an acute infectious disease that is caused by the yellow fever virus. Humans may acquire the infection through the bite of an infected mosquito. The "yellow" in the name is explained by the jaundice that affects some patients.
  
• Yellow fever is now a serious public health threat again. It has caused large epidemics in Africa and the Americas. The WHO estimates that there are some 200,000 cases of the disease and 30,000 deaths each year. Immunization is the most effective method to prevent the disease.
  
• An outbreak of yellow fever can go undetected because the signs and symptoms of yellow fever are similar to viral hepatitis, malaria, Ebola/haemorrhagic fever and other viral haemorrhagic fevers.
  
• In urban yellow fever, large epidemics can occur when migrants introduce the virus into areas with high human population density. Aedes aegypti mosquitoes carry the virus from person to person. These outbreaks tend to spread outwards from one source to cover a wide area.
  
• Symptoms of the disease do not appear until 3 to 6 days after infection. After this "incubation" period, the infected person usually enters an "acute" phase that is characterized by fever, muscle pain (with prominent backache), headache, shivers, loss of appetite, nausea and/or vomiting. Most patients improve and their symptoms disappear within three to four days in this acute period, however, 15 percent enter a "toxic phase" within 24 hours with fever, jaundice, abdominal pains and vomiting. Half of the patients in the toxic phase die within 10 to14 days. The remainder will recover without significant organ damage.
  

Source: World Health Organization

GAVI Secretariat, c/o UNICEF, Palais des Nations, 1211 Geneva 10, Switzerland
Tel: 41 22 909 5019 Fax: 41 22 909 5931 Email: Gavi@unicef.org