From profs Michael Kremer and Jeffrey Sachs.

Sir, Dr Leonard Lerer (Letters, May 18) misunderstands both our proposal and the epidemiology of malaria in sub-Saharan Africa.

We are advocating that donor countries commit in advance to buy an effective malaria vaccine at a reasonable market price if and when such a vaccine is developed. The promise today would create market incentives for the needed research and development effort. No money would be spent under our proposal until an effective vaccine actually comes to market, so our approach would not take current funding away from alternatives approaches to malaria control.

Suppose that such a vaccine is developed. Would it be cost-effective for an international fund to buy it at a price range of $10-$40 per fully immunised child? Direct calculations suggest that an effective vaccine in that price range would be among the most cost-effective public health interventions of any kind (see details at www.cid.harvard.edu/malaria), rivaling that of oral rehydration therapy. Not surprisingly, many leaders in public health have pressed the urgency and appropriateness of vaccine development.

Dr Lerer's treatment of malaria epidemiology is misinformed. Africa is not suffering from a "current malaria epidemic", as he claims. In large parts of the continent, malaria is hyper-endemic, with the majority of the population carrying the parasite, and individuals receiving hundreds of infectious mosquito bites a year. While mosquito control and other environmental approaches may indeed limit the disease burden, these will not, by themselves, control the disease sustainably. In many cases, malaria transmission is so high that mosquito control is simply not feasible.

In any event, a combination of environmental control, drug therapy and vaccine development will almost surely be appropriate. Each part of an overall strategy will strengthen the others.

Michael Kremer, Massachusetts Institute of Technology, and Jeffrey Sachs, Center for International Development, Harvard University, 79 John F. Kennedy Street, Cambridge, MA 02138 US