From time to time, Dr. Michael L. Goldfein, a pediatrician on Martha's Vineyard, faces this predicament: a parent who does not want a child vaccinated. In the eyes of Goldfein and other pediatricians, immunizations represent one of medicine's greatest triumphs. But for as many as 5 percent of parents who come to his practice, these life-saving injections are seen as risky and potentially harmful. "It's a real challenge and it's quite frustrating because these people really don't listen to the facts," said Goldfein, who has practiced on Martha's Vineyard since 1977. "They feel that they have the right to decide whether or not their children can get vaccinated in spite of all the evidence."