Last winter after an eleven-year-old child with measles visited a California theme park, the Centers for Disease Control traced at least one hundred and twenty-five other cases to the outbreak. About a quarter of those contracted the disease through secondary infection; that is, they were not at the park, but came in contact with someone who was.

The potential for that kind of ripple effect is what made health officials on the Vineyard jump to action when an out-of-state child who had played all over the Island came down with measles here last week. By press time, no new cases had been reported, though the incubation period had not quite passed.

The Vineyard is especially vulnerable to an outbreak, having the highest percentage of unvaccinated schoolchildren of any Massachusetts county. Eleven per cent of the students included in a state public health survey have not received the MMR vaccine, which guards against mumps, measles and rubella, compared with a statewide average of five per cent.

The state requires children going into preschool or childcare to have had one dose of MMR and two doses before they enter kindergarten. However, exemptions are given for medical and religious reasons, and the latter category has become something of a catchall for people who object to vaccinations for any reason.

It would be one thing if the risk were simply to those children whose parents had opted — for whatever reason — not to vaccinate them. But among the most vulnerable populations are infants who are too young to receive vaccines and the immune compromised who are too sick. And measles, which killed thousands when it was commonplace in the fifties, is still potentially deadly, Dr. Jeffrey Zack, chief of emergency medicine at the hospital, reminded community members this week.

Young parents who think they are doing their children a favor by withholding vaccinations did not live through the horrors of smallpox, polio and other once-common childhood diseases. With luck, the Vineyard will escape an outbreak of measles this summer, but those who would risk giving to a foothold here clearly don’t understand what the greater good means.