"145 Figure 11 . The MMR vaccine contains live attenuated strains of measles, mumps and rubella viruses. Note that each of the memory cells and the T cells and B cell s generated in an adaptive immune response are specifically directed against just one type of pathogen or a very closely related strain. Vaccination with MMR vaccine is only protective against measles, mumps and rubella infection. Despite decades of effort , medical science has so far been unable to produce effective vaccines against a number of important infectious diseases, including HIV/AIDS and other sexually transmitted infections (although trials of several candidate vaccines are underway). Vaccines ag ainst malaria have shown some potential, but they only protected about half the vaccinated children in large - scale African trials. There are no effective vaccines against infections caused by any other single - celled or multicellular parasites. Some vaccine s are effective only for a relatively short time, e.g. cholera vaccines give protection for only around six months. Others, such as the tetanus vaccine, need repeated booster doses. Annual influenza vaccinations are offered to vulnerable groups in the popu lation because influenza viruses alter their antigens over time. Opposition to vaccination Another major challenge is parental opposition to vaccination because of concerns about vaccine safety. Some parents are understandably anxious about taking a healthy child to be given a vaccine derived from infectious agents. Most vaccines are injected, which can be a frightening and painful experience for a child. Of even greater concern is that some children experience a negative reaction to a vaccine, most o ften inflammation around the injection site and/or a mild fever lasting a day or two, but rare instances of more severe reactions such as convulsions sometimes occur. Hesitancy about vaccinating a child is increased if parents have no experience of the dis ease the vaccine prevents, because it has become rare in communities with high vaccine"