An increasing rate of whooping cough infections statewide has public health and school leaders concerned, especially now that California requires an immunization before children can enter school in the fall.
The concern is heightened in Contra Costa County, where whooping cough rates may exceed last year's epidemic rate statewide.
In Alameda County, where the infection rate is not as severe, more than 100,000 school children have yet to comply with a new law requiring vaccination. Nearly that many in Contra Costa need the shot.
California had almost 9,300 cases last year and was the only state nationwide to declare a whooping cough, or pertussis, epidemic. In 2010, 547 infants younger than 6 months were hospitalized, and 10 died.
"We're very concerned," said Erica Jenssen, Contra Costa's health department immunization coordinator.
She said that 78 cases have been reported this year in Contra Costa and 733 statewide. Three infants were hospitalized, although all recovered.
"But that means it's still out there in the community," Jenssen said.
Alameda County reported 48 cases among its 1.4 million residents, and Los Angeles County -- with nearly 10 million residents -- had 88.
Last year marked the highest incidence of pertussis in California since 1958, according to a study this month in Primary Care Reports.
Pertussis is a highly contagious bacterial disease. It typically begins with a cough and runny nose for up to two weeks, followed by weeks to months of rapid coughing fits that may end with a whooping sound as the person breathes in. Fever is rare.
Very young infants may not even cough, as their lungs haven't developed enough to expel the mucous the disease creates in the lungs.
"That's where the fatality issues start to happen," said Jeff Dimond, a spokesman with the Centers for Disease Control and Prevention.
Most infants with whooping cough require hospitalization, and infants account for most of the deaths, although the mortality rate is low.
In the pre-vaccine era, pertussis was the nation's leading cause of death from communicable disease among children younger than 14 years. The advent of a childhood whooping cough vaccine in the early 1940s cut the rate by 90 percent.
A gradual increase in infection rates began in the 1980s. Health researchers are uncertain why, and better diagnosis and surveillance may explain some of the increase.
The concerns some parents have about childhood vaccines is not a factor in the spike, said Alison Patti, a CDC spokeswoman.
The endemic disease, often mistaken in adults for a bad cough, bronchitis or other condition, peaks in three- to five-year cycles, according to the CDC.
In a health breakthrough, a combination vaccination against tetanus, diphtheria and pertussis, called Tdap, was approved in 2005.
The CDC recommends that young children receive five shots of a childhood combination vaccine called DTaP, which protects against the same diseases. The shots start at 2 months and are completed with a fifth shot between 4 and 6 years. But the immunization wears off by middle school.
For that reason, California now requires students entering grades 7 through 12 this fall to show proof they've had the pertussis booster vaccine. As long as a child had a Tdap shot after age 7, he or she fulfills the new requirement.
For the rest, children can get the shot at doctors' offices or at pharmacies. County public health departments offer clinics for uninsured or underinsured residents.
Despite schools' attempts to notify families, many have not yet complied, said Peggy Marshburn, spokeswoman for the Contra Costa County Office of Education,
"Parents are not paying a whole lot of attention to this," she said. "The concern is that come school time, there's going to be this whole big rush and they won't be allowed back into the school until they have it. "... It's a very serious disease."
Roughly 130,000 students in Alameda County need to comply with the new law, said Sherri Willis, a spokeswoman with the Alameda County Public Health Department. "We still have a long way to go," she said.
The 38,000-student Oakland school district has been actively pressing the issue.
An automated phone message went out to every home, informing families of the immunization requirements, said Troy Flint, a district spokesman. The district also has been promoting free clinics held by the Alameda County Public Health Department.
The booster shot also prevents an unpleasant disease among older people, sometimes called the "100-day cough," Patti said.
But the booster shot is largely meant to protect infants from infected adults.
Parents and caretakers are advised to keep anyone without the booster shot away from infants, particularly those younger than 6 months. That's because the first whooping cough vaccinations require several months to take effect.
"We want to create what's called a 'cocoon of immunity' about that child," Dimond said.
Staff writers Theresa Harrington and Katy Murphy contributed to this report. Contact Suzanne Bohan at 510-262-2789. Follow her at Twitter.com/suzbohan.
For uninsured or underinsured residents, contact your local public health department for information on free or low-cost whooping cough vaccinations.
In Alameda County, go to www.acphd.org/pertussis or call 510-267-3200.
In Contra Costa County, go to http://cchealth.org/topics/pertussis.
Some pharmacies, such as those at Safeway and Rite-Aid, also offer whooping cough booster vaccines.