seven serotypes (also in rank order) were 14, 6, 5, 1, 19, 9, and 23. Thus, frequency of infection with pneumococcal types 1 and 5 appears to distinguish developing-world populations from those in developed nations.

This was not always the case. Some 40 years ago, pneumococcal serotype 1 was the most common serotype in the United States. Likewise, serotype 5 was once much more prevalent in the United States than it is now. This sort of shift, or “secular change,” is not limited to the dramatic alteration in U.S. pneumococcal distribution. Temporary, year-to-year fluctuations in serotype prevalence have been documented in many developing countries.

Although estimates of serotype prevalence in the developing world are fairly good, data from certain regions (particularly South Asia and much of sub-Saharan Africa) are less complete than those from others. A number of efforts are underway to fill these data gaps. One such initiative, funded by the Rockefeller Foundation and USAID, is tracking invasive pneumococcal and haemophilus disease in six sites in India. The SIREVA project of the Pan American Health Organization is undertaking similar research, focused only on S. pneumoniae, in Latin America and the Caribbean.

In the India project, some 8 percent of 1,000 blood cultures collected initially were positive for S. pneumoniae, and preliminary subtyping indicates types 1 and 5 are the most common on the subcontinent. Data gathered over the past several years from Pakistan, Bangladesh, and India indicate that, compared with the United States, pneumococcal disease disproportionately strikes the very young, those less than 5 months old. Therefore, a vaccine that is immunogenic only in older infants may prevent disease in U.S. infants while having much less of an impact in the developing world. This reinforces the need to design vaccine trials in the developing world that look specifically at efficacy in young infants.

With respect to meningococcal meningitis, lack of solid data on disease incidence outside of the established market economies has played out in a different way. In a number of recent outbreaks—in Ethiopia (1989), Brazil (1989–1990), Cameroon (1991–1992), and Mongolia (1994)—actual disease burden has been poorly quantified. Health officials have often reacted by initiating mass immunizations with polysaccharide vaccines. Although necessary under the circumstances, such actions cannot prevent death and illness during the first few weeks of an epidemic, and they have tended to divert funds and personnel from routine public health services. The availability of an effective conjugate meningococcal vaccine that could be given to young children would probably prevent such scenarios from occurring.

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