It's official: there's a whooping cough epidemic in California

After weeks of warnings, the California Department of Public Health confirmed Friday that cases of pertussis – or whooping cough – have reached epidemic proportions in the state.

As of this week, there have been more than 3,400 cases of whooping cough reported in the state this year. That's more than in all of last year.

Statewide, the rate of the disease is highest in Sonoma, Napa and Marin counties. In the Los Angeles area, the highest rates of disease are in Long Beach, which has reported almost 20 cases per 100,000 people, and Ventura County, which has reported about 11 cases per 100,00 people.

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Infants too young to be vaccinated are most vulnerable to severe and fatal cases of whooping cough. This year, two babies have died of the disease, and two-thirds of pertussis hospitalizations have been in babies four months or younger, according to the state health department.

As a result, state health officials are urging all pregnant women to get vaccinated against the disease during their third trimester. This allows mothers to transfer pertussis antibodies to their infants, which should protect them until they can be vaccinated at six weeks old.

Older children and adults – especially those around newborns – should also be vaccinated, according to current recommendations.

Whooping cough peaks every three to five years, and the last epidemic was in 2010. Experts say this cycle occurs because getting the disease does not guarantee immunity in the future, and the pertussis vaccine has a waning efficacy over time.

RELATED: Higher rates of older kids with whooping cough worries health officials

"Unlike some other vaccine-preventable diseases, like measles, neither vaccination nor illness from pertussis offers lifetime immunity," Dr. Ron Chapman, director of the state health department, said in a statement. "However, vaccination is still the best defense against the potentially fatal disease."

Whooping cough symptoms vary by age, according to public health officials. For a child, a typical case starts with a cough and running nose. When the cough worsens, the child may have a rapid coughing spells that end with a "whooping" sound, like in this CDC video.

In adults, pertussis may cause a cough that persists for several weeks. Infants may not have typical symptoms, and no apparent cough, but parents may see episodes when the infant’s face turns red or purple.