California is in the midst of its worst outbreak of whooping cough in a half-century. More than 2,700 cases have been reported so far this year — eight times last year's number at this point. Seven of the victims, all infants, have died.

And here's what really worries pediatricians like USC's Harvey Karp: Doctors thought they wiped out whooping cough when they developed vaccines decades ago.

The disease hits young children hardest, especially ones who are not vaccinated or who have not yet built up full immunity. The prescribed vaccination regimen begins with a shot at two months and continues until children are 5 years old. For many children, it can take that long for complete immunity to develop — and until then, they're vulnerable.

The California epidemic has raised plenty of questions about the role of vaccination and the increasing numbers of parents who decide not to vaccinate their children. California's Department of Public Health cites three schools in the state where 80 percent of parents have signed a "personal belief exemption" to keep their children from being vaccinated.

That's part of what's behind this epidemic, Dr. Karp tells NPR's Guy Raz. "And it's in part because the immunity of people who were immunized earlier has waned," he adds.

In fact, Karp estimates that 75 percent to 90 percent of whooping cough cases occur in teenagers and adults, for whom it is not deadly. Most adults may experience only a lingering cough. The danger occurs when adults and teenagers — whose immunity wanes around 12 years old — contract whooping cough and unwittingly give it to infants.

There is a booster shot for adults, but it only became available in 2005. Many adults are not even aware that they should get one every 10 years.

"Doctors — especially pediatricians — aren't doing a good enough job," Karp says. "When you go into the doctor's office for your child's care, your pediatrician should be telling you that you should be getting immunized as well."

Karp acknowledges that vaccination is a difficult issue for parents.

"They're very busy, working hard, working double jobs — it's really a burden to have to be a parent and then also have to be a biochemist, to read the literature and decide what vaccines are good and what vaccines aren't good," he says.

In the background, he says, parents see a frightening increase of autism, coupled with reports of a potential link to vaccinations.

"All of that has stopped parents in their tracks from wanting to take any chance that they were going to expose their child to something that might be dangerous," Karp says. "I totally understand that. The good news is — we have a large body of information now to show zero association between vaccines and autism."

"But," he adds, "we do need to try to help parents figure all of this information out so that they can make the best decision for their kids."