In contrast, DTP, HBV and inactivated polio vaccine (IPV) seem to exert a non-specific detrimental effect on child mortality, although the findings on DTP were considered controversial by a recent review. Current studies indicate that the negative effect of DTP may be neutralized by a subsequent measles vaccination. It is striking that all the vaccines with a non-specific beneficial effect are live, whereas the vaccines with an apparently harmful effect are killed. Results from animal studies have shown that attenuated live vaccines tend to induce a Th1 response and offer better protection against severe disease than the corresponding inactivated vaccines, which tend to induce a Th2 response. So far, very few studies have examined whether these effects differ between male and female animals. One study reported that BCG-vaccinated female mice were better protected against malaria parasites than male mice (31). There is therefore an urgent need to conduct studies that can help uncover the immunology behind the non-specific effects.