Costing vaccines versus bednets In a typically elegant analysis comparing the cost-effectiveness of vaccines and insecticide impregnation of mosquito nets for the prevention of malaria, Patricia Graves recently estimated that the costs per death averted for a vaccine would be US$252 compared to US$ 711 with impregnated nets 1 . The estimates for the mosquito-net costs were based on the costs of re-impregnation with insecticide every six months. Recently, an insecticide- impregnated wash-proof mosquito net has been developed that is sold at the same price as an untreated net. The costs, therefore, of using such a net in a similar analysis would be nil. As such, the conclusion that malaria- vaccine research and development should be the highest priority for investment might need to be reconsidered. Of course, there are a number of other factors (both pro and contra) that need to be considered in the arguments and analyses. For example, the actual amount of investment needed to develop a vaccine (which probably runs into billions rather than millions of US$) was ignored. Similarly, it was assumed that everyone already possessed a net (which is a considerable capital cost), that all nets were re-impregnated every six months (when in fact re-impregnation rates are generally less than a third) and that people used their nets all the time. Other perceived end-user benefits from using a net [such as a good night’s sleep (the GNS factor), controlling bed bugs, scabies or other diseases, such as filariasis and kal-azar] were also not included in the overall evaluation. Graves was worried in case impregnation programs became so entrenched that it might be difficult to switch to more effective strategies (such as a vaccine program) when they become available. Many control methods (including vaccines and mosquito nets) are, however, complementary and, in an integrated program, they would all have their place. We can already faintly hear the voices of dissent talking about the possibilities of insecticide resistance developing when large numbers of wash-proof nets are in use. This issue could perhaps be addressed by persuading the manufacturers to incorporate two insecticides on the nets. This should reduce the likelihood of resistance developing against either insecticide in much the same way that combined therapy should work against the parasite. Ultimately, strategies to avoid malaria will be in the hands of the people affected by the disease. The best way of ensuring that all of the available techniques are used appropriately is that people know the likely personal costs and benefits of the different strategies they might adopt. As such, the role of education, particularly for the younger generation, cannot be over emphasized. J.D. Charlwood* S.E. Clarke C. Bøgh Danish Bilharziasis Laboratory, Jaegersborg Allé 1 D, DK-2920 Charlottenlund, Denmark. *e-mail: dc@bilharziasis.dk Reference 1 Graves, P.M. (1998) Comparison of the cost- effectiveness of vaccines and insecticide impregnation of mosquito nets for the prevention of malaria. Ann. Trop. Med. Parasitol. 92, 399–410 Human immune response to Trichuris trichiura In a recent review, Mao et al. 1 write that ‘Some helminth infections induce strong Th2 immune responses, similar to those involved in atopy, while others, such as Trichuris trichiura, appear to induce Th1 rather than Th2 responses’. This interpretation is based on research by Else et al. 2 What Else et al. have shown in laboratory mice is that some host strains are resistant to infection by T. muris in that they develop acute infections only, whereas others are susceptible and remain chronically infected 2,3 . The immune status of resistant mouse strains was characterized during infection by T-helper cell type 2 (Th2) polarization, while that of susceptible strains was primarily a Th1-cell profile. Seen in the context of the estimate that over 900 million people globally are infected by T. trichiura, mostly in developing countries 4 , the nature of the cytokine profile in humans is possibly very important. This is because it is in the same communities where trichuriasis and other helminthic infestations are so highly prevalent, that epidemics of HIV infection and TB are burgeoning out of control. Evidence is accumulating that resistance to HIV and Mycobacterium tuberculosis is enhanced when the immune profile is dominated by Th1 cytokines, and impaired when Th2 responses predominate 1,5,6 . In children chronically infected by T. trichiura, the host reaction in the colonic mucosa is characterized by high concentrations of IgE, many degranulating mast cells and release of histamine, as compared to controls 7 . Circulating IgE is also elevated. These findings are not typical of a Th1 profile. At present, there is probably more evidence that a Th2 response in humans to T. trichiura antigen might be the norm, rather than a Th1 reaction. Further research needs to be carried out. John E. Fincham Medical Research Council, PO Box 19070, Tygerberg 7050, South Africa. M iles B. M arkus* Parasitology Research Programme, University of the Witwatersrand, Private Bag 3, Wits, Johannesburg 2050, South Africa. *e-mail: milesm@iafrica.com References 1 Mao, X-Q. et al. (2000) The link between helminthic infection and atopy. Parasitol. Today 16, 186–188 2 Else, K.J. et al. (1992) Cellular immune responses to the murine nematode parasite Trichuris muris. II. Differential induction of Th-cell subsets in resistant versus susceptible mice. Immunology 75, 232–237 3 Else, K.J. and Grencis, R.K. (1991) Helper T-cell subsets in mouse trichuriasis. Parasitol. Today 7, 313–316 4 Bundy, D. and Peto, R. (1998) Anthelmintic treatment for infestation with Trichuris trichiura. Lancet 352, 1935 5 Adams, J.F.A. et al. (1999) Decline in total serum IgE after treatment for tuberculosis. Lancet 353, 2030–2033 6 Bentwich, Z. et al. (1999) Can eradication of helminthic infections change the face of AIDS and tuberculosis? Immunol. Today 20, 485–487 7 Cooper, E.S. et al. (1991) Immediate hypersensitivity in colon of children with chronic Trichuris trichiura dysentery. Lancet 338, 1104–1107 TRENDS in Parasitology Vol.17 No.3 March 2001 http://parasites.trends.com 1471-4922/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. 121 News& Comment Letters