These immune system alterations can significantly limit the capacity to Cell Cycle inhibitor maintain previously acquired protection against vaccine antigens or respond to new vaccine stimulations . Moreover, there seems to be a significantly increased risk of severe adverse events, particularly when live attenuated vaccines are administered . No data are available concerning the progressive decline in the titres of antibodies against vaccine antigens during chemotherapy, but all of the evaluations made towards the end of, or after cancer treatment have shown that a substantial proportion of children have lower concentrations than those considered
to be protective or lower than those found in healthy children. Table 1 summarises the residual protection provided by the most widely used pediatric vaccines , , , , ,
, ,  and . In the case of the vaccines for which the correlates of protection have been established, it is important to note that all of the studies show that protection is reduced, but the percentage of children whose antibody levels are lower than the limit of protection varies. This is probably related to factors such as the intensity and duration of treatment, the type of cancer, the time of evaluation, the type of vaccine, the methods used to assay antibody levels, and the age of the patients. Nilsson et al. found that younger children are at higher risk of losing specific antibodies, probably because the developing B lymphocyte pool (especially bone marrow plasma cells) is more vulnerable during chemotherapy in younger patients . Moreover, lower than protective learn more antibody
levels against vaccine antigens have been found for several months after the discontinuation of chemotherapy , , , , , , ,  and . Nevertheless, these findings do not indicate that all of the children in these conditions are unprotected against a specific disease because further exposure to vaccine antigens as a result of revaccination leads to a secondary immune response in most of those who have apparently lost their immunity, thus showing the persistence of an adequate immune memory . However, the fact that revaccination fails to evoke protective levels of specific antibodies in a minority of cases indicate that immune recovery is not complete much . The lowest responses are usually seen in younger children, probably because the time needed to reconstitute memory lymphocytes is longer than in the older ones . It is also possible that younger children can have lower vaccine-antigen specific antibody concentrations at completion of treatment and poor responses because they did not have all or any of the childhood vaccines prior to commencing chemotherapy. Despite these age-related differences, absolute lymphocyte counts generally return to normal values within 3 months  and .