San Diego County, battling a deadly outbreak of hepatitis A, is postponing an outreach campaign to provide the second of two inoculations against the contagious liver disease until a national shortage of the vaccine is resolved, the county’s chief public health officer said.

“Our goal is to get that vaccine in as many arms as possible for that first dose,” said Dr. Wilma Wooten, who is leading the fight against an epidemic that has ravaged unsanitary homeless encampments in San Diego County for the past year, sickening 544 people and killing 20 of them as of Nov. 6.

Nurses and other county medical workers are fanning out across the most at-risk areas to offer onsite inoculations, and if they run into people who are due for the second shot, they will still give it to them, said Wooten, Public Health Director at the county’s Health and Human Services Agency.

The two hepatitis A vaccinations, considered the best way to control the spread of the virus, should be administered six months apart. The first shot is the most important, Wooten said, because it protects people 90 to 95 percent of the time against the virus that causes the disease. The second shot raises the protection level to “close to 100 percent,” she said.

So far, 90,735 people have received vaccinations in San Diego County — most of them the first of the two-shot series, according to the county’s health agency.

The San Diego outbreak, and a number of others in California and across the United States, have generated a spike in demand for hepatitis A vaccine and put a squeeze on supplies, according to the federal Centers for Disease Control and Prevention. Unexpectedly high demand worldwide has constrained availability outside the U.S. as well, the agency said.

Merck & Co. and GlaxoSmithKline, the two companies with approval from the Food and Drug Administration to sell the vaccine in the United States, said they have been hard-pressed to keep up with the demand and are working to boost their production.

The effects of hepatitis A can range from mild to fatal. In addition to the deaths in San Diego, an outbreak of the illness in Michigan has sickened 486 people and killed 19, as of last Friday, according to the Michigan Department of Health & Human Services.

Los Angeles and Santa Cruz counties are also fighting the illness, and infections linked to California’s outbreaks are spreading to homeless people in Utah and Arizona, and to men engaging in gay sex in Colorado, the CDC said. In New York City, health officials are confronting a smaller outbreak, mostly among gay or bisexual men.

The deadly nature of the epidemics in San Diego and Michigan worries public health officials the most, said Dr. Noele Nelson, a CDC specialist in hepatitis vaccine research and policy. “The number of deaths in the Michigan and San Diego outbreaks are quite high from what we’ve seen in the past,” she told members of the CDC’s Advisory Committee on Immunization Practices at a late-October meeting in Atlanta.

Hepatitis A is typically spread through the ingestion of fecal matter from an infected person — even in microscopic amounts. That can happen when people carrying the virus fail to wash their hands after defecating and then contaminate objects, food or water used by others. It can also spread through sexual contact.

On Oct. 13, California Gov. Jerry Brown declared a state of emergency in an effort to increase the state’s supply of adult hepatitis A vaccine. The declaration allowed the state “to immediately purchase additional vaccines directly from manufacturers and coordinate distribution to people at greatest risk in affected areas,” the California Department of Public Health said.

Before Brown’s emergency declaration, the department had distributed nearly 80,000 doses of the vaccine obtained through a federal vaccine program, but those supplies were insufficient, it said.

Merck and GlaxoSmithKline sell the hepatitis A vaccine in pre-filled syringes and less costly single-dose vials.

Pamela Eisele, a Merck spokeswoman, said the unexpectedly sharp rise in demand for the vaccine has limited availability of the company’s vaccine this year.

Single-dose vials of the company’s VAQTA brand vaccine have been on backorder since May and weren’t available until last week, Eisele said. The company expects prefilled syringes to be unavailable until the first quarter of next year, she added.

Likewise, GlaxoSmithKline has been struggling to fill orders for its Havrix brand of the vaccine.

“It’s unprecedented, and it’s very large what’s happening,” said Robin Gaitens, a spokeswoman for the company. GlaxoSmithKline only recently received a shipment of prefilled syringes and has a “limited supply of vials in stock,” she said.

“We will continue to work with CDC, the California Department of Public Health, which is coordinating vaccine orders and distribution on behalf of the counties, and our private customers in California to help address the needs in the state,” Gaitens added.

San Diego County’s Wooten said that despite the supply constraints nationwide, the county now has enough vaccine on hand to give the first injection, but not the second, to those most at risk of contracting the virus — namely, the county’s homeless people, illicit drug users and the professionals who provide care to them.

The biggest challenge posed by the San Diego outbreak is getting the vaccines to people in the transient homeless population, Wooten said. To help address that, the county has hired about 100 temporary nurses to supplement the public health nursing staff, nurse volunteers from local hospitals, paramedics and homeless outreach workers who are on the front lines of the vaccination effort.

The city of San Diego has also been taking actions to curb the spread of the infection. In addition to spraying the streets in infected areas with a bleach solution, it has so far installed 78 hand-washing stations and 16 portable toilets for the homeless.

The city has also opened a public campsite with tents, sinks and restrooms for up to 200 people in a municipal operations yard downtown, said Katie Keach, spokeswoman for the city.

Amy Gonyeau, chief operating officer of the Alpha Project, a homeless outreach organization that is operating the campsite for the city, said 181 people, including 40 children, are living there so far.

Whether those efforts are making a dent in the spread of the hepatitis A infection isn’t yet known.

“San Diego has reported fewer cases per week over the past two weeks than it reported previously,” the CDC’s Nelson said at last month’s advisory committee meeting in Atlanta. “But it’s too early to say this indicates a downward trend in the overall outbreak.”

This story was produced by Kaiser Health News, which publishes California Healthline, an editorially independent service of the California Health Care Foundation.