Who is to blame for the spread in California of whooping cough, which has killed nine infants this year?

Unimmunized adults and teenagers are one major factor, health officials say.

“We don’t think it is the coverage level in babies and toddlers that is the problem,” but the lack of vaccination in adults and teens, Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases at the U.S. Centers for Disease Control and Prevention, recently told reporters.

As of Tuesday, state officials reported 4,223 confirmed, probable and suspected cases of whooping cough, also known as pertussis, in California. The latest week included 206 new cases, compared with 183 the previous week.

With no indication that the spread is slowing, physicians and community groups plan to hold a news conference Thursday in downtown Los Angeles urging adolescents — specifically Latinos — to get immunized. According to a federal survey conducted in 2008, just 40.8% of people ages 13 to 17 had received the Tdap shot, which protects against tetanus, diphtheria and pertussis.

The numbers are even lower for adults. Ken August, a spokesman for the California Department of Public Health, cited a separate 2008 survey that found that only 6% of U.S. adults said they had received Tdap.

“Parents don’t realize that this is a huge problem,” said Dr. Aliza Lifshitz, an internist at Cedars-Sinai Medical Center in Los Angeles and editor of a Spanish-language health website, vidaysalud.com.

Parents are generally very good at taking babies and young children in for shots, Lifshitz said, but typically forget to take them back when they are older. The booster shot is important because “with whooping cough, after five years, the vaccine tends to lose its protection,” she said.

Although Latino teenagers have a higher-than-average rate of immunization for their age group, organizers said a Spanish-language campaign is key to reaching a community that does not respond to English-language campaigns. Eight of the nine infants who have died this year in California were Latino.

The introduction of the Tdap shot, approved by the U.S. Food and Drug Administration in 2005, came after scientists’ view of the disease evolved over the decades.

“In the days of the pre-vaccine era, 93% of the cases were recognized in patients 10 years of age or less. Nevertheless, it was circulating in adults, but it just wasn’t recognized,” said Dr. James Cherry, a UCLA pediatrics professor and expert on pertussis.

Stopping the spread of whooping cough among adults is crucial, health officials said, because it is undiagnosed adults — especially mothers and fathers — who often infect newborn babies.

California health officials now recommend the Tdap inoculation for everyone 7 years and older, including the elderly, who is not fully immunized, and especially pregnant women and anyone who will have contact with them and their babies.

ron.lin@latimes.com

Times staff writer Thomas H. Maugh II contributed to this report.