A major new problem has just surfaced that is causing concern among U.S. health officials. A new report from the Centers for Disease Control and Prevention indicates that the United States will run out of the vaccine for yellow fever as early as next month, meaning that travelers who need the shot may not be able to go anywhere.

A manufacturing problem appears to be behind the shortage of the only version of the vaccine that is licensed in the U.S., and is often a requirement for countries where mosquito spread diseases are a problem. They may require proof of vaccination in order to enter the country, meaning people without it will be turned back.

It’s been a century since yellow fever was eradicated in the United States, and it’s not a disease people are routinely vaccinated for here. But in many countries where tropical diseases thrive, it’s still a problem. It can cause jaundice, abdominal pain, vomiting, internal bleeding, and organ failure.

“Recent manufacturing problems resulted in a shortage of the only U.S.-licensed yellow fever vaccine,” the CDC said in a statement. “This shortage is expected to lead to a complete depletion of yellow fever vaccine available for the immunization of U.S. travelers by mid-2017. CDC, the Food and Drug Administration (FDA), and Sanofi Pasteur are collaborating to ensure a continuous yellow fever vaccine supply in the United States. As part of this collaboration, Sanofi Pasteur submitted an expanded access investigational new drug (eIND) application to FDA in September 2016 to allow for the importation and use of an alternative yellow fever vaccine manufactured by Sanofi Pasteur France, with safety and efficacy comparable to the U.S.-licensed vaccine; the eIND was accepted by FDA in October 2016. The implementation of this eIND protocol included developing a systematic process for selecting a limited number of clinic sites to provide the vaccine. CDC and Sanofi Pasteur will continue to communicate with the public and other stakeholders, and CDC will provide a list of locations that will be administering the replacement vaccine at a later date.”