Cases of whooping cough - an infection that can kill otherwise healthy infants - continued to climb in Arizona this year.

Through Dec. 15, the number of probable and confirmed cases of whooping cough is up 67 percent from 2010 and up 5 percent from 2011. The number has climbed steadily since 2007. One infant in Arizona died from whooping cough earlier this year. About one in every 100 infants hospitalized with whooping cough will die, said Dr. Karen Lewis, medical director of the Arizona Department of Health Services' Immunization Program Office.

State officials blame multiple factors for the continued rise in whooping cough, including better awareness and reporting, improved testing and an increasing number of parents who are not vaccinating their children against the highly contagious disease, also called pertussis.

"Anybody who is unvaccinated can help in the spread," Lewis said. "I took care of hospitalized children with infectious diseases for 15 years, and I am concerned when anybody gets sick with a vaccine-preventable disease. I am very worried about whooping cough spreading to infants because I have had to take care of children with whooping cough who have died despite doing everything I could do."

A switch from what's called a "whole cell" pertussis vaccine to a more purified "acellular" version has also contributed to the increased number of cases. The switch, made in the 1990s, appears to be causing the childhood pertussis vaccine to wear off faster than in the past, Lewis said. The change was made to reduce side effects. That's why it's so important for both adults and adolescents to get a pertussis "booster" shot.

But the number of adults who have received the federally recommended whooping cough booster remains far too low, particularly among health-care workers, Lewis said. Very few hospitals require their employees to have it, and as a result only 20 percent of health-care workers have received the booster. And less than 10 percent of the general adult population has received the booster, leaving the infants they come in contact with extremely vulnerable.

The incidence of whooping cough has not decreased percentagewise as much as other vaccine-preventable diseases in recent years, Lewis said.

"Vaccination doesn't give you 100 percent protection against the disease, but you are much less likely to get infected, and you are much less likely to spread it to other people," she said. "That's why people choosing not to get vaccinated are making not just a personal choice, but a public health choice."

The rise in Arizona whooping cough cases mirrors a national trend. As of Dec. 8, more than 39,000 cases of whooping cough - double the number in 2011 - had been reported across the U.S. Nationwide, 16 people died from whooping cough this year.

Arizona's whooping cough rate of 13.5 cases per 100,000 people this year is higher than the national average of 11.6 per 100,000, but much lower than some states including Colorado, Washington and Vermont, which reported epidemic levels of whooping cough this year.

Before pertussis vaccines became widely available in the 1940s, about 200,000 U.S. children got sick with it each year and about 9,000 died.

An Arizona Daily Star investigation in May found that one in three Arizona schools last year had kindergarten classes with vaccination rates so low that children were vulnerable to outbreaks of vaccine-preventable diseases. The investigation also showed that more mid- to high-income parents are choosing not to vaccinate their children.

Those findings were confirmed this month when public health researchers from the University of Arizona published a study in the journal Vaccine that tracked a rise in the number of so-called "personal belief exemptions" that parents sign to excuse their children from getting immunized. In some schools, as many as 20 percent of the children were unvaccinated, the researchers found. Overall exemption rates by school varied from zero percent to 68 percent.

The highest rate of vaccine exemptions was in Arizona charter schools, both the Star and the UA study found. The researchers also found that per capita, Arizona has the highest number of charter schools in the country. The UA researchers also found clusters of unvaccinated children in the area of northeast Yavapai County, the city of Sedona and in Colorado City.

In line with other national studies, the predominant characteristics of the schools with high exemption rates are a higher proportion of white middle- to upper-class students, the study said. Historically, lower vaccination rates were more often observed in lower-income families who lacked access to care, but that has changed.

The UA study said concern by parents that their children are getting too many vaccines coupled with a desire for a more "natural" lifestyle have fueled the rising rates of personal belief exemptions. They also found influences in popular culture that have fueled the rise among more upscale families.

But it's extremely important for public health officials to identify the clusters where unvaccinated children live because of the health risks they present to the rest of the population. Among other examples, the study cites a 2008 measles outbreak in San Diego that was started by an unvaccinated child at a charter school where 11 percent of the children were exempt from immunizations. The outbreak eventually spread to 48 people.

"The reason we chose to study exemption rates in Arizona is because they have tripled in the last decade, and we wanted to know the characteristics of those who choose not to vaccinate so we can target public health messages," said study co-author Elizabeth Jacobs, an associate professor of epidemiology at the UA's Mel and Enid Zuckerman College of Public health.

Jacobs said the next steps are to create programs that demonstrate the safety and benefits of vaccination.

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Probable and confirmed cases of whooping cough in Arizona

*2012: 913

2011: 867

2010: 546

2009: 277

2008: 218

2007: 210

*through Dec. 15 Source: Arizona Department of Health Services

Whooping Cough Vaccines

The shots that prevent whooping cough are the DTap (diphtheria, tetanus and pertussis) in infants and children and Tdap (tetanus, diphtheria and pertussis) in preteens and adults.

The pediatric formulations usually have three to five times as much of the diphtheria component as the adult formulation. That is indicated by an uppercase "D" for the pediatric formulation and a lowercase "d" for the adult formulation. The amount of tetanus toxoid in each of the products is equivalent, so it remains an uppercase "T."

Getting vaccinated:

• Pertussis vaccines are recommended for people of all ages. Infants and children should get five doses of DTap for maximum protection. A dose is given at 2, 4 and 6 months, at 15 through 18 months, and again at 4 through 6 years. A booster dose of Tdap is given to preteens at 11 or 12 years of age.

• Any adolescents or adults who didn't get Tdap as a preteen should get one dose. Getting Tdap is especially important for pregnant women and others who care for infants. You can get the Tdap booster dose no matter when you got your last regular tetanus booster shot (Td). Also, you need to get Tdap even if you were vaccinated as a child or have been sick with pertussis in the past.

Contact reporter Stephanie Innes at or 573-4134.