PLEASANTON — The vibrations from launching fireworks at the Alameda County Fair probably disturbed two nests of yellow jackets Friday night and resulted in about 100 people being stung, fair and fire officials said Saturday.

Minutes after the fair’s fireworks show began about 9:15 p.m., a steady flow of spectators suffering from insect stings made their way to the first aid booth, Alameda County Fire Department spokeswoman Aisha Knowles said.

The medical staff at the fair treated nearly 100 people in less than an hour, she said. Several people were treated for multiple stings, but no one suffered an allergic reaction, and no one needed to be taken to the hospital. The insect attack victims ranged in age from 6 months to 60 years.

After examining the area of the grandstands where the incident occurred, officials determined fairgoers had been attacked by a swarm of yellow jackets.

Their nests were found around midnight Friday at the base and near the top of a palm tree at the north end of the grandstands. The yellow jackets were exterminated, according to a statement by fair spokeswoman April Mitchell.