A 28-year-old woman from San Juan Capistrano is the first Orange County resident under 65 to die of the influenza virus during the 2013-14 flu season, county health officials said Thursday.

The woman, whose name was not disclosed, contracted the H1N1 strain of the flu, which made the 2009-10 season particularly deadly. The federal Centers for Disease Control and Prevention estimated that between 43 million and 89 million people were infected with that strain during the 2009-10 season, causing between 8,870 and 18,300 deaths.

There were 57 deaths in Orange County that season from H1N1, a nasty strain that hit young people unusually hard.

Strains of the virus tend to change slightly from year to year, returning and then subsiding, said Dr. Matthew Zahn, medical director of the Division of Epidemiology and Assessment program with the county’s Health Care Agency. The good news is that the seasonal flu vaccine guards against three strains of the virus, including H1N1. That strain has been included every year since the 2010-11 season.

In 2009, a separate H1N1 vaccine was rushed to the market, and health officials labored to get the public to take two flu shots instead of one.

The inclusion of H1N1 in every seasonal vaccine since 2010-11 has undoubtedly saved lives and prevented millions of infections, Zahn said.

“One piece of good news is the CDC is looking at the H1N1 viruses out there, and they’ve found that this virus is not significantly different than what’s been out there the last few years,” he said. “So it matches well with the vaccine.”

The San Juan Capistrano woman had “underlying medical conditions,” Zahn said, without specifying. The very young and old, and people who have existing illnesses or other conditions that can compromise their immune systems, are most susceptible to severe illness from the flu, which is why it’s important for anyone 6months and older to get vaccinated, health officials say.

Flu cases are notoriously difficult to quantify, because most people who get the virus don’t seek medical attention. Influenza-related deaths in people 65 and older aren’t officially reported by medical centers.

Zahn said people can decrease their risk of catching the flu by getting vaccinated, as well as washing their hands frequently. If you’re sick, stay home from work or school to avoid infecting others.

Some people might be misled into thinking that, since the push for flu vaccinations – at the doctor’s office or the pharmacy – began months ago, flu season itself is mostly over. Zahn said the county has seen an uptick in flu cases the past two weeks, and HCA said so far this season eight cases have been classified as “severe,” meaning the patient had to be hospitalized in intensive care. That compared with three severe cases, including one death, at the same time last year.

Zahn said this flu season could stretch through May.

“The disconnect that could happen is there’s a difference between flu vaccine season and flu season,” he said.

“September and October is a wonderful time to get your flu vaccination, because it’s going to cover you for the season that comes. But most years, most of the disease comes after the new year, January and on,” Zahn said.

After inoculation, it takes about two weeks for the vaccine to take full effect.

Contact the writer: 714-796-2221 or lhall@ocregister.com