The New York City Department of Health and Mental Hygiene (DOHMH) is investigating three new cases of hepatitis A virus (HAV) infection since December 2018 and appears to be gearing up for a major public health push to help prevent an outbreak, Outbreak News Today reports.

The cases involved people experiencing homelessness or using drugs and could signal that a nationwide outbreak of the virus has spread to the city. So far, 18 states have reported HAV outbreaks among people experiencing homelessness or using drugs.

Officials say the Department of Health, the New York City Department of Homeless Services and local providers are working together to protect all New Yorkers and have stepped up efforts to offer preventive hepatitis A vaccines to at-risk populations.

This isn’t the first time in recent years New York City has ramped up its vaccination efforts in the face of a potential outbreak. In 2017, DOHMH issued a similar warning after 51 people—mostly men who have sex with men—tested positive for the virus.

Hepatitis A is highly contagious, and cases of the liver infection can range from mild (lasting only a few weeks) to severe (lasting several months). Death from the virus is not common, but the mortality rate in the outbreaks seen across the country has been high 3 percent—likely because of concurrent illnesses those experiencing homelessness may have.

To learn more about hepatitis A and its risks, click here.