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Nearly 50 students at Harvard-Westlake School have been recently diagnosed with whooping cough, in an outbreak that has forced school officials to send students home at the first sign of illness.

But all of the sick students had been vaccinated against the disease, according to school officials. In fact, all 90 people who have recently come down with pertussis — the official name for whooping cough — in Los Angeles County this year had been immunized against it, according to county officials.

It turns out that four years after someone receives the booster shot in about the seventh grade, the vaccine’s protection nearly vanishes, endangering high schoolers such as those at Harvard-Westlake, said Dr. James Cherry, a UCLA expert on pediatric infectious diseases.

“It is not surprising at all,” Cherry said of the recent cases.

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