The following is excerpted from the
Third Annual Conference on Vaccine Research
[Infectious Diseases Conference Summaries - © 2000 Medscape, Inc.]

Eradication of Polio, Measles, and Hib

Katherine M. Neuzil, MD
Department of Medicine,
University of Washington School of Medicine, Seattle

The 20th century may be viewed by future historians
as the time when we began to eradicate diseases.

Global Eradication of Polio

Poliomyelitis is an old disease. Clay pictures and hieroglyphs suggest that polio occurred among the ancient Egyptians. In the United States, the most vivid and well-described epidemics occurred in the 20th century. The frequency of paralytic polio increased in the first half of that century, reaching its peak in the 1950s. Subsequently, the introduction of polio vaccine and mass campaigns resulted in the eradication of wild-type polio in the United States; the last case of indigenously acquired wild-virus infection was reported in 1979. In 1988, the World Health Assembly resolved to eradicate poliomyelitis globally by the end of 2000.

Obstacles and Solutions

Several obstacles to achieving the goal of polio eradication by the end of 2000 remain. The project is costly, and Dr. Hull anticipates a shortfall of $300 million.

The polio virus must be kept cold for optimal immunogenicity, and this may be difficult, particularly as workers try to reach children in remote areas of the world. Each vial of vaccine is now equipped with a thermosensitive spot that indicates if the vaccine has been heated to the point of inactivation. If the spot changes color, then the vaccine is not used.

Finally, countries at war provide a unique obstacle to the international efforts. However, with the aid of political leaders, truces have been declared so that NIDs can proceed.

Despite the obstacles mentioned above, the global eradication of polio is near. Dr. Hull and others expect certification to be finished by 2005. At that point, we will have eradicated a second global scourge. And the resources used to eradicate polio can then be used to remove other diseases, such as measles, from the earth.

Eradication of Measles from the Americas

Those in the developed world have lost sight of measles since universal vaccination was introduced. Nevertheless, measles is a major cause of morbidity and mortality among the world's children. It accounts for 10% of global mortality from all causes among children younger than 5 years, accounting for approximately 1 million deaths annually. In addition, measles continues to be a major cause of blindness worldwide.

Developing a Susceptible Pool

Measles is a highly contagious disease, and without vaccine, every child will acquire measles. When vaccination is introduced into a population, the interepidemic period increases. Dr. Ciro A. de Quadros, Director of the Division of Vaccines and Immunization, PAHO, gave the following example: Assuming vaccination coverage of 90% and vaccine efficacy of 90%, only 81% of susceptible children within each birth cohort are immune to measles after a vaccination campaign. Over time, the susceptible population grows, resulting in large outbreaks of measles. The goal of the enhanced measles eradication program is to break the cycles and interrupt transmission.

Current Progress

What have the results of this enhanced measles elimination campaign in the Western Hemisphere been? From 1987 to 1995, every country except the United States had a "catch-up" campaign. In 1996, there were 2,109 cases of measles in the Western Hemisphere, the fewest ever reported. Most measles cases in the United States are now caused by international importations, and there have been fewer importations from Latin American and Caribbean countries to the United States in recent years.

Eradication of Hib

Haemophilus influenzae type b (Hib) causes serious invasive disease among previously healthy children younger than 5 years old. Throughout most of the world, Hib has been the leading cause of bacterial meningitis in children. It is also an important cause of bacteremia, pneumonia, and other serious infections of the joints, skin, and respiratory tract. Invasive Hib disease occurs almost exclusively in children younger than five years old, with the peak incidence occurring between 6 months and 2 years of age. The incidence of invasive Hib disease in the prevaccine era was approximately 20-90 cases per 100,000 children younger than 5 years old. Approximately half of these cases were meningitis, and half were other invasive diseases.

Vaccine Efficacy

The Hib vaccine is an inactivated semisynthetic derivative of type b polysaccharide conjugated to a protein carrier to stimulate immunity in infants. It is over 90% efficacious in preventing invasive disease. Since the introduction of vaccine into the United States, there has been a 99% decline in the number of reported Hib invasive disease cases among children younger than 5 years old, according to Dr. Joel I. Ward of Harbor-UCLA Medical Center. Dramatic decreases in Hib disease incidence have also occurred in other countries in North and South America and Europe.

Summary

Since the global elimination of smallpox, the world community has set goals for the eradication of other vaccine-preventable diseases limited to humans. Polio eradication is within sight, and measles eradication in the Americas is nearly reached. With continued worldwide efforts, more diseases can be eliminated, and the health of the global population can be improved.