Many California schools are barring students from classes if they haven't received a whooping cough vaccine, in accordance with a state law passed last September.

The law, which requires that all students entering grades seven through 12 be vaccinated, took effect for the 2011-2012 school year. The state legislature passed a 30-day extension over the summer to give students more time to comply, but thousands of students still have not gotten the vaccine -- and, depending on the school district they're in, some of them can't attend class until they do.

Starting on Thursday, the San Francisco school district sent home students who came to school without proof of vaccination or a signed parental personal belief exemption, in which parents can certify that they have chosen not to vaccinate their child because of personal opposition to vaccinations. A spokeswoman for the school district told The Huffington Post that about 2,000 students, or 10 percent of students in the district, had not complied with the requirement. She added that cost should not be a factor, because the district is holding free vaccination clinics.

The law was passed in response to the state's worst outbreak of whooping cough, or pertussis, in more than 60 years, which sickened more than 9,000 Californians and killed at least 10. Public-health officials believe the outbreak was related to an increase in parents who refuse to vaccinate their children because they are afraid of side effects.

Most infants receive the DTaP vaccine, which protects against whooping cough, tetanus and diphtheria, but adolescents need a booster shot to stay protected. People who do not get the initial vaccination or the booster shot put at risk not only themselves but also the people they come into contact with. If a lot of students are not vaccinated, outbreaks can occur, as in California.

If one of them gets it and they're all together, you now have a whole pool of susceptible people, John Talarico, chief of immunization for the California Department of Public Health, told The Huffington Post.

Mandatory vaccination programs are controversial, because many people, especially those who support small-government policies, believe it is not the government's place to require parents to vaccinate their children. On the national stage, for example, Republican presidential candidate Rick Perry has taken a lot of heat for his attempt as governor of Texas to require sixth-grade girls to be vaccinated against HPV, a virus that can cause cervical cancer.

But it is not just a partisan issue. Nathan Fletcher, a Republican assemblyman in San Diego, is a staunch supporter of the requirement. This is not an academic or philosophical discussion, he told The Huffington Post. Children have died as a result of this. We took very seriously our obligation to protect children, so I think school districts need to take seriously the obligations to comply with it.

Initial symptoms of whooping cough resemble the common cold, but as the disease progresses, it can cause violent coughing fits that lead to vomiting and exhaustion. The coughing fits can last up to 10 weeks. Most people eventually recover, but there can be serious complications, especially for infants. Half of all infants with whooping cough suffer from apnea, or slowed or stopped breathing. One in five get pneumonia, one in 100 have convulsions, one in 300 develop encephalopathy, or brain disease, and one in 100 die.

The symptoms of whooping cough are usually milder in teenagers and adults, but complications can include weight loss, rib fractures or passing out from coughing fits. About two percent get pneumonia.

Whooping cough is extremely contagious, especially in the first week or two, when symptoms are mild -- meaning people may go about their daily routines and infect many others before they realize they are sick.

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