For the past three years, San Diego County has had the highest occurrence of whooping cough, or pertussis, than any other county in the state, according to data from the California Department of Public Health.

In a snapshot report published Oct. 21, 2018, San Diego had 526 confirmed cases of pertussis so far this year.

Los Angeles County, which has three times of the population of San Diego, had 251 and Orange County, which has a comparable population to San Diego, had 138.

The higher number doesn’t necessarily mean San Diego is more susceptible to the disease than the rest of the state, the county's public health officer, Dr. Wilma Wooten, said.

“San Diego has great collaboration with and reporting from local providers, so I feel that has contributed to our higher numbers,” she said.

Last year, San Diego had 1,163 confirmed whooping cough cases, while Los Angeles had 558 and Orange County had 198.

In 2016, San Diego also led the state with 395 cases, ahead of L.A.'s 309 and Orange County's 69.

One of the possible reasons that San Diego has a higher occurrence is because the county has a higher number of unvaccinated and under-vaccinated children or children who are behind on their vaccination schedule.

For the last two school years, the percentage of students entering kindergarten in San Diego County with all the required vaccines lag below the state average.

Around 93.2 percent of children entering kindergarten in the 2017-18 school year in San Diego are fully vaccinated, according to the CDPH’s Kindergarten Immunization Assessment report. The state average is 95 percent.

San Diego was also below the state’s average in the 2016-17 school year at 94.7. Both L.A. and Orange County were at or above the state average for the last two school years.

“If you're trying to look at what's driving the outbreak and then if you look at what parts of the county have higher rates, you'll see that it's the parts of the county that historically have higher exemption rates for school for vaccines,” said Eric McDonald, San Diego deputy public health officer. “That's also true of other counties in California … [outbreaks are] higher in places with higher [exemption] rates.”

In California, all children are required to have pertussis, tetanus and diphtheria vaccines before entering kindergarten, though some children are conditionally allowed to attend school if they are behind on their vaccines. Children are also required to receive a booster vaccine before entering the seventh grade.

Over the past two decades, there has been an anti-vaccine movement based on the faulty and discredited research of Dr. Andrew Wakefield, who linked vaccine to autism. His claim has been debunked by numerous studies and recent studies suggest that there is a genetic component to autism.

That, however, did not stop parents from requesting personal belief exceptions to not vaccinate their children. In 2016, the state of California banned such exemptions for children entering school. The effect of the law was almost immediate.

In San Diego, the number of personal belief exemptions dropped to nearly zero for the 2017-18 school year after there were 655 the year before.

The same pattern arose in Orange County. In the 2017-18 school year, Los Angeles, however, had two personal belief exemptions.

The number of permanent medical exemptions, however, rose. Between the 2016-17 school year and the 2017-18 school year, San Diego saw a 0.3 percent increase, Los Angeles a 0.4 percent increase and Orange County a 0.1 percent increase.

It was unclear if the increase was because parents whose children can’t get vaccinated because of medical reasons are now requesting permanent medical exemptions because the personal belief exemption was easier to get.

The concern for McDonald though is the number of pre-teens and teenagers with pertussis.

While whooping cough is a bad cough for older children, it is deadly for infants. So far this year, there have been 233 cases in children ages 10-17, according to the San Diego County Pertussis Case Reports published Nov. 29, 2018.

At the same time, 120 children between 6 months old and 3 years old were diagnosed with whooping cough.

“What's driving the numbers of cases are actually middle school and high school students who have persistent cough,” McDonald said. Because pertussis is highly infectious, it’s passed on to younger siblings, he said.

A San Bernardino infant in July became the first baby to die from whooping cough in California since 2016. The U.S. Centers for Disease Control and Prevention recommends vaccinating children at two months of age. Pregnant women in their third trimester are also encouraged to get vaccinated against pertussis.

“What's contributing to the numbers of cases under the age of six months is that it's been challenging for us to get all pregnant women to be immunized in every pregnancy in the third trimester,” McDonald said. “Every pregnant woman in every pregnancy [should] get a vaccination in a third trimester in order to prevent bad outcomes, for infant pertussis in particular.”

From 1965 to 2002, the number of whooping cough cases in the U.S. dropped below 10,000, but since 2003, that number has risen, according to the CDC.

There were nearly 18,000 cases of the disease reported in 2016, the most recent year of the data available from the CDC. The number was actually a decrease from the preceding years.