The San Francisco Chronicle reports that roughly 2,000 of the city’s middle and high school students were barred from attending class Thursday because they had yet to be immunized against pertussis, commonly known as whooping cough, and they can’t return until they get the shot.

The action was taken under a new state law that requires all seventh- to 12th-graders, in both public and private schools, to get the pertussis vaccine by the first day of school.

California is no stranger to the ravages of the illness. The state experienced last year the worst whooping cough epidemic it had seen in 50 years. By January 2011, the state Department of Public Health reported more than 8,000 cases and 10 infant deaths.

The students had apparently been given fair warning. District officials said the lockout of unvaccinated kids came only after phone calls to homes, numerous written reminders, in-school announcements, free community vaccination clinics and a 30-day extension on the vaccination deadline, according to the Chronicle.

Dr. William Schaffner, chairman of preventive medicine at Vanderbilt University in Nashville, Tenn., and an ardent supporter of vaccination efforts, said that despite the lockout of unvaccinated students, the impact on these kids’ class schedules should be minimal, because free vaccinations were made available immediately to those who had yet to receive them.

“The California example shows that if you do the right thing and provide the resources to get it done quickly and appropriately, it works,” he said. “And it works to protect the individual as well as the community.”

In addition to being highly contagious, whooping cough is a grueling illness. Its advanced stages are characterized by a severe, hacking cough followed by a high-pitched intake of breath, or the “whoop” that gives the disease its nickname.