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    The Whooping Cough Epidemic: Why Now? continued...

    Once the symptoms of the bacterial respiratory infection set in -- including the strong cough that leaves patients making a ''whooping'' sound as they try to catch their breath -- young infants in particular can deteriorate quickly, developing a high white blood cell count, respiratory distress, and deadly pneumonia.

    All the California infants who died were under 3 months old and thus not fully protected against pertussis. The five-dose series of pertussis vaccine to protect against whooping cough typically begins at age 2 months, but sufficient protection isn't achieved until about the third dose, at around 6 months, experts say.

    To help protect infants, it is particularly important they stay away from someone who is not immunized, whether it is a child who never received the vaccine or an adult who has waning immunity because they did not get a booster shot.

    In California, which allows parents to exempt their children from vaccinations for philosophical reasons, the rate of unvaccinated children in 2009 was 2%, CDC figures show. Some parents, fearing side effects from vaccines, depend instead on the concept of ''herd immunity.” Because many others are vaccinated, their own child's chances of catching the disease theoretically are lower.

    ''Those who don't vaccinate contribute'' to the epidemic, but the cyclical nature of the disease is mainly to blame, says James Cherry, MD, a professor of pediatrics at the David Geffen School of Medicine at the University of California Los Angeles, who has studied pertussis for 30 years. He reviewed the fatal California cases for the state.

    Public health officials agree that unvaccinated children are just one factor that has played a role in the epidemic. Others include:

    • The cyclical nature of pertussis. Pertussis typically reappears every three to five years. “The main thing is the cycle,” Cherry says. The last cyclical peak in California, Harriman says, was in 2005. Fueling the fire, the disease is also highly contagious, Harriman says.
    • Unprotected adults. ''Immunity to pertussis wanes with increasing time since vaccination, so adolescents and adults need to boost their immunity," says Thomas Clark, MD, MPH, medical officer and epidemiology team leader of the CDC's National Center for Immunization and Respiratory Diseases.
    • Difficulty in diagnosing. Because the first symptoms may be mild, whooping cough is not always easy to diagnose, says Brian Johnston, MD, director of the emergency department at White Memorial Medical Center in Los Angeles, where some children have been hospitalized with pertussis, a rare occurrence there. The organism, Bordetella pertussis, can be ''hard to culture," he says. "It is extremely difficult, the diagnosis. You have to be clinically suspecting it all the time," and be prepared to prescribe antibiotics -- the treatment for whooping cough -- even though doctors have been schooled in general not to over-prescribe them.
    • Mutations in the bacteria. Pertussis bacteria may be mutating and outwitting the vaccine, according to a Dutch researcher who published his findings in 2009 in the journal Emerging Infectious Diseases.

    Not to blame, Harriman says, are immigrants. Nine of the 10 California fatalities were Hispanic infants. That could be linked to the size of Hispanic households, typically larger than those of other ethnic groups, she says. More household contacts increase the chances of someone there being exposed to whooping cough.

    "Pertussis has never left the U.S., ever,” Harriman says. “We don't need anyone to bring it here, it's here."