Denver health officials are offering free hepatitis A vaccines in an effort to curtail an outbreak of the contagious liver infection that has popped up in the city.

So far, four individuals have been diagnosed with hepatitis A, including three within the past four weeks. All of the cases involved people experiencing homelessness, according to a news release from Denver Public Health.

The hepatitis A cases in Denver are connected to a larger outbreak that has taken place nationwide, including in southern Colorado. Since 2016, there have been “sizable outbreaks of hepatitis A” in urban areas across the U.S., said Dr. Seth Foldy, director of epidemiology, informatics and preparedness at Denver Public Health, at a news conference on Monday.

He said health officials have been preparing for an outbreak in Denver since 2017.

The outbreak has affected people experiencing homelessness and those with substance-use issues, in part because these populations have less access to health care, vaccinations and sanitary conditions. The opioid crisis and other substance use that involves sharing injection and smoking materials are also contributing to the outbreak, Foldy said.

“The general public is at very low risk in this situation,” he said.

While Denver has sporadic cases of hepatitis A each year, officials noted that the first case tied to the outbreak appeared in June. Health officials didn’t rule out the possibility of more cases.

A person can get hepatitis A by ingesting the virus through objects, food or drinks that are contaminated by stool from an individual already infected. It can also spread through sex and sharing drugs. Symptoms include fatigue, low appetite, stomach pain and nausea, according to the Centers for Disease Control and Prevention.

The four individuals diagnosed with the virus were hospitalized and have recovered, Foldy said.

The Denver cases come as 25 states have reported outbreaks of hepatitis A, according to the CDC.

In early May, Colorado health officials announced a hepatitis A outbreak that began in October and already had resulted in 52 cases of the contagious liver infection.

As part of that effort, public health agencies in the state had provided almost 6,000 vaccinations to at-risk populations. The outbreak affected people experiencing homelessness and those with substance-use issues, along with individuals who are incarcerated and the contacts of those at-risk, said the Colorado Department of Public Health Environment in a news release.

Denver health officials are spearheading an effort to make sure at-risk populations are vaccinated, including giving individuals shots on the streets or at parks, at a rate of 700 a month, Foldy said.