Because most children today are healthy, children suffering from serious conditions such as cancer or heart disease are relatively few in number compared with the number of adults who have such conditions. As one indicator of children’s good health, children aged 0 to 19 years accounted for 29 percent of the total U.S. population of 272.7 million in 1999 but only 2 percent of all deaths—about 55,000 deaths for children compared with more than a half million deaths for adults aged 20 to 64 years and 1.8 million for those age 65 years and over (NCHS, 2000).
To cite another example, the American Cancer Society has estimated that approximately 1.3 million new cases of cancer would be diagnosed in 2003, of which an estimated 9,000 would involve children ages 0 to 14 years (ACS, 2003). For any specific cancer in children, the numbers are much smaller. For example, each year approximately 300 children are diagnosed with retinoblastoma (a tumor of the retina), 1,100 are diagnosed with astrocytoma (a brain tumor), and 2,400 are diagnosed with acute lymphoblastic leukemia (based on data from 1977 to 1995) (Ries et al, 1999). Moreover, as treatments for childhood cancer have improved and rates of cancer-free survival have increased, researchers find fewer children with relapsing disease who are potentially available for the study of new chemotherapeutic agents.
Exceptions exist to the “small-numbers” phenomenon. Each year approximately 75,000 babies are born very prematurely at 31 weeks gestation or earlier, and more than 350,000 babies have low or very low weights at birth (Martin et al., 2002). (For various reasons, however, many of these newborns will not be appropriate to include in research.) Asthma is a growing problem among children, and the increase in childhood obesity has become a major concern. A number of less serious conditions, such as otitis media, acne, and mild allergies, are quite common.
Although the numbers of children affected by some of these illnesses are greater than the numbers affected by rare genetic disorders, not all children with a disorder or condition will be eligible or able to participate in clinical trials. For any one research location, the numbers available for a study are usually quite low.
For many pediatric studies, the relative scarcity of potential study participants means that it often takes considerable effort and some creativity to enroll and retain sufficient numbers of children who meet the criteria for study participation. It may also mean that studies must extend for quite long periods just to secure enough participants. One recently published article on the prevention of fungal infections in children and adults with chronic granulomatous disease reported that it took 10 years to enroll 39 participants, most of whom were children at the time that the study started (Gallin et al, 2003).