California health officials on Friday reported 835 more whooping cough cases in the last two weeks, for a total of 5,393 since Jan. 1 -- and there's little sign of the epidemic abating.

The whooping cough epidemic is now more than double the 2,532 cases recorded for all of 2013, and so far this year has claimed the lives of three infants.

Overall whooping cough, or pertussis, rates are highest for babies under 1 year old, followed by adolescents and teens from 10 to 17 years old, with the peak age at 15 years old, according to the new report. Several Bay Area county health department officials say teenagers in their regions continue to be hardest hit.

Dr. George Han, Santa Clara County's deputy health officer, confirmed the majority of the county's pertussis cases involve teenagers. Infants, especially those under 6 months, however, remain most at risk for serious complications, hospitalization and death, Han said.

As a result, both state and local efforts this year have been focused on vaccinating pregnant women in their third trimester, as they can pass that immunity to their unborn babies, protecting them until they can be vaccinated.

"This underscores the need to vaccinate all infants, pregnant women and family members who live with infants, including preteens and teenagers," said Han.

Paul Leung, Contra Costa County's immunization program manager, said another baby in that county was just hospitalized with pertussis, bringing the total to five so far this year.

"That's more than the total number of babies hospitalized for all of 2013,'' said Leung, who also emphasized the importance of vaccinations for pregnant women during the third trimester.

Health officials attribute the current whooping cough epidemic to its three-to-five-year cycle; the last epidemic occurred in 2010. They also point to a corresponding use of "acellular'' pertussis vaccines, which cause fewer reactions than the whole-cell vaccines that preceded them. But those vaccines don't protect as long, lasting only three to five years.

Officials say the disease may also have spread in some areas because some Californians have chosen not to get vaccines for themselves or their children due to religious or personal beliefs.

A state law that took effect Jan. 1 requires parents who exclude their children from immunization requirements to submit a signed state form proving that they received information about the risks and benefits of vaccines from a health care professional, who also must sign the form.

But parents who cite religious reasons are exempt from that requirement.

Contact Tracy Seipel at 408-920-5343. Follow her at Twitter.com/taseipel.

2014 whooping cough cases as of July 8:
Alameda County: 173
City of Berkeley: 40
Contra Costa County: 222
* Marin County: 168
Monterey County: 75
* Napa County: 124
San Benito County: 2
San Francisco: 27
San Mateo County: 59
Santa Clara County: 269
Santa Cruz County: 59
Solano County: 101
* Sonoma County: 589
* Marin, Napa and Sonoma counties have the highest whooping cough rates in the state
Source: California Department of Public Health