California public health department officials have reported 835 more whooping cough cases in the last two weeks, for a total of 5,393 since January 1 – and there’s little sign of it abating even though July is traditionally the peak month of the disease.

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 one year old, followed by adolescents and teens from 10 to 17 years old, with the peak age at 15 years old, according to the report issued Friday.

Dr George Han, Santa Clara County’s Deputy Health officer, confirmed the majority of the county’s pertussis cases involve teenagers.

Infants, especially under six months, however, remain most at risk for serious complications, hospitalisation 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 immunisation program manager, said another baby in that county was just hospitalised with pertussis, bringing the total to five so far this year.

“That’s more than the total number of babies hospitalised for all of 2013,” said Leung, who also emphasised 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 January 1 requires parents who exclude their children from immunisation 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.