Sonoma County's incidence of the disease, formally known as pertussis, is more than double even that of the next closest counties — Marin and Tehama — which reported rates just above 28 per 100,000 population, the state agency said.
"Pertussis is widespread in Sonoma County," Karen Holbrook, the county's deputy health officer, said Friday.
Even this week, the reports continued to come in, reaching 356 confirmed cases by Thursday, she said.
"I can't say if we've peaked or not," Holbrook said. "It's too early to tell. But there has been a steady increase."
Pertussis, known as "whooping cough" because of the distinctive gasp that often follows a coughing fit in those affected, historically runs in three- to five-year cycles, peaking every few years, as it did in 2010.
In what was the worst epidemic in more than 60 years, California reported 9,159 cases that year, including 10 deaths — all infants.
Two babies have died in California this year, the first fatalities since 2010, state health officials said. At least 77 patients have been hospitalized, most younger than 3 months old, the state reported.
Sonoma County has had no deaths, but total cases for 2014 already are higher than the 246 reported in the county in all of 2010, Holbrook said.
It appears we're in for another peak year, Ron Chapman, director of the state Department of Public Health, said in a news release Friday.
Even young people up to date on recommended childhood immunizations are at risk of infection because of the waning protection the vaccine offers, health officials said.
Anecdotal reports suggest those getting sick this year are suffering symptoms that are less severe than in the past, however, Holbrook said.
Though the vaccine is imperfect and diminishes in potency over time, research indicate it is 80 to 90 percent effective, Holbrook said.
According to one study, seven in 10 children remain fully protected five years after completing the childhood series, while three in 10 are partially shielded, she said.
Infants too young to be immunized against whooping cough are so much more vulnerable to severe, potentially fatal symptoms that California health officials are now recommending pregnant women receive vaccine boosters in the third trimester to help build some resistance in their offspring.
Anyone who works with or lives with infants should be vaccinated, as well, Holbrook said. Babies can be vaccinated as early as 6 weeks old, though the full series of childhood vaccinations is typically not complete until 4 to 6 years of age.
Oddly, the age range most widely affected this year in Sonoma County is 10-to-14, with kids 15-to-19 in the second largest group, Holbrook said.
Statewide, 90 percent of the cases have been in children under the age of 18, with 32 percent of them in ages 14-to-16, the California Department of Public Health said.
A change in the vaccine formulation in 1997 may be the reason, a spokesman for the state health agency said.
The newer vaccine causes fewer local reactions, but its protection against pertussis wanes more quickly than the one that went into widespread use in the 1940s, drastically reducing the nation's incidence of pertussis, which once caused tens or hundreds of thousands of cases in the nation each year, according to the Centers for Disease Control.
The bacterial infection is spread through respiratory droplets passed through coughing or sneezing in or near someone's face. It is not airborne in the sense that it lingers in the air, Holbrook said.
But it is highly contagious nonetheless between people who share the same household, classroom, office or some other space for an hour or more.
Holbrook said 12 Sonoma County schools had current outbreaks. She would not identify the schools.
Over the past five years, at least, California's rates have risen in the summer months, suggesting the worst may be to come.
"Infectious diseases are always difficult to predict," said Corey Egel, a spokesman for state public health agency, "but if historical patterns hold, cases will likely continue to rise."
(You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com.)
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