This story has been updated. See change below.

by Joanne Faryon | KPBS and Kevin Crowe | inewsource

Listen to the story on KPBS.

San Diego health officials may be closer to knowing what led to last year’s whooping cough outbreak – the worst in more than 60 years.

Scientists from the Centers for Disease Control and Prevention (CDC) will present their preliminary findings from an investigation into California’s whooping cough outbreak at a national vaccine conference in Washington, D.C. this week.

KPBS and inewsource at SDSU spent months investigating the epidemic and found many of the people getting sick had been immunized.

More than 9,000 people tested positive for whooping cough, or pertussis, last year in California. San Diego had the second highest number in the state with more than 1,100 positive cases in 2010.

“It does look like the peak of pertussis outbreak was in mid-2010, but we’re still having continued cases,” said Dr. Eric McDonald, San Diego County deputy public health director.

Whooping cough, or pertussis, is a highly contagious upper respiratory illness that in adults and older children can mimic a bad cold. In infants the disease can be fatal. Last year, 10 California babies, two from San Diego, died from the disease.

Infants under 2 months old are too young to be immunized and children under 6 require five doses to be considered protected.

An analysis of the most current state data by KPBS and inewsource shows a little more than half of all the people diagnosed with pertussis in California last year had been immunized.

The data also show more than half of all cases in the state were in children 10 and under, with infants less than a year old accounting for 17 percent of cases.

Of the 7- to 10- year-olds diagnosed with pertussis, slightly more than half were up-to-date with their immunizations.

CDC investigators were in San Diego County late last year to gather data on positive cases, McDonald said.

“The CDC has done a specific investigation looking at vaccine effectiveness. They came here to San Diego to investigate some of our cases from 2010,” McDonald said.

Investigators said county doctors were especially diligent about diagnosing positive cases and that could have contributed to the fact San Diego County had a relatively high number of cases per capita.

McDonald also noted the percentage of kindergarten children entering San Diego schools whose parents have chosen not to have them immunized increased from 2.6 to 3.1 percent in 2010.

This week, the CDC will present some of what it learned from last year’s outbreak in a presentation entitled: “What’s an epidemic like this doing in a highly-vaccinated community?”

A previous version of the story quoted Dr. Eric McDonald as saying the personal belief exemption rate for San Diego County was 2.3 percent in 2009. The statistic he provided was 2.6 percent.

Whooping Cough in California by County

Click on a county to view the number of cases of pertussis reported and rate of incidence for 2010.

The California Department of Public Health included pertussis cases with onset from January 1 through December 31, 2010 as reported to the department by local health jurisdictions.

Darker colors on the map indicate higher rates per capita.

Joanne Faryon is a freelance reporter and former inewsource and KPBS reporter.