Use of vaccine trials to estimate burden of disease.

Vaccine trials, the most informative way of determining the efficacy of a vaccine, can also provide valuable information about the burden of disease. The burden of Haemophilus influenzae type b (Hib) remains a major barrier to the use of Hib vaccines, especially in Asia. Recent studies in Indonesia and Bangladesh have used vaccine-trial designs, with known effective vaccines, to estimate the vaccine-preventable burden of Hib disease in those communities. New vaccines against pneumonia and diarrhoeal diseases are usually directed at only one of various causes of the syndrome. In the case of pneumonia, it is very difficult to determine the aetiology in most cases, so the vaccine trial offers a means of determining the burden of vaccine-preventable diseases. This is particularly important for pneumococcal vaccines as serotype replacement may reduce the effectiveness of the vaccines in the field. This approach would underestimate disease burden if vaccines were found to have an impact on syndromes other than those against which they are directed, and might lead to errors in estimation if there were erroneous assumptions about the efficacy of the vaccine against the condition under investigation.