As a direct result of Hib immunizations, only approximately 300 cases of Hib currently occur each year in the United States, compared with nearly 20,000 before the conjugate vaccines were available. But the success of Hib vaccination has not resonated worldwide, according to Heikki Peltola, MD, professor of infectious diseases at the University of Helsinki, Helsinki, Finland, who presented these findings here at the recent 38th Interscience Conference on Antimicrobial Agents and Chemotherapy.
The realization of the burden of Hib infections in some European countries did lead to significant vaccination activity when the conjugate vaccines became available in the 1980s, but immunization activities have since declined.
Conjugate vaccines have marginally impacted Hib disease in privileged regions in Europe, primarily because only affluent people and countries can afford these costly vaccine. This is a restrictive factor and open-minded research is needed in the countries where Hib diseases may still be rampant, Peltola said.
The yearly case numbers of various Hib disease manifestations - at least 17,000 among the 43 million children 0-4 years of age currently residing in Europe - has declined only to around 10,000. Heavily populated countries like the Ukraine, Poland and Italy virtually have not initiated large-scale vaccinations at all, Peltola said.
The incidence of the classical Hib manifestations at age 0-4 years was approximately 44 cases per 100,000 people in northern Europe and somewhat less in central and southern Europe. In contrast, Switzerland and the Netherlands reported rates up to 100 cases per 100,000 people.
However, these rates were still one-third lower than the overall U.S. incidence of Hib disease, said Peltola, also a member of the Infectious Diseases in Childreneditorial board.
About 35,000 cases - only a 2% decrease - have been prevented globally with immunization because the vaccine is often too expensive for developing countries, Peltola said.
In the world population of approximately 5.7 billion, 631 million (11%) are younger than 5 years. These children are the most vulnerable and at the highest risk for Hib infections. Hib causes at least 50% of all bacterial meningitis cases worldwide; the remaining cases are caused by meningococci and pneumococci.
Although meningitis represents approximately 40% to 60% of classical Hib disease, it is not the only illness caused by Hib. Additional manifestations linked to Hib include acute epiglottiditis, articular infection and septicemia.
Throughout the world, Peltola said, pneumonia is the most common problem associated with Hib, although it is mostly forgotten from calculations because of problems in proving the etiology. Many H. influenzae respiratory infections are not a result of Hib but other serotypes, but it is likely that 20% to 25% of severe pneumonia is caused by Hib, he explained. When pneumonia is added to the calculations, the number increases significantly. Peltola estimates that more than 2 million severe Hib infections occur worldwide per year and suggested Hib be considered as a major global health problem.
In fact, he added, Hib kills approximately 30% of those infected and leaves another 30% who survive the disease with permanent sequelae - usually hearing impairment after meningitis.
Because of lacking incidence data, especially in developing countries, overall worldwide estimations of Hib cases are difficult. Data from more than 30 countries (10 representing the developing world) suggest an overall incidence for classical Hib diseases, excluding pneumonia, at 72 cases per 100,000 in the developing world and 57 per 100,000 in the more developed countries; respectively, at 0 to 4 years of age.
There are currently four Hib conjugate vaccines currently approved by the Food and Drug Administration for use in the United States, the first of which was approved in 1987. Hib vaccine was added to the routine childhood immunization schedule in 1990.
For your information:
- Peltola H. Only slightly declined burden of Haemophilus influenzae type b (Hib) disease in Europe. Paper G-54. Presented at the 38th Interscience Conference on Antimicrobial Agents and Chemotherapy. Sept. 24-27. San Diego.