Both drugmakers have acknowledged issues. A Merck spokesperson said the company is has had manufacturing constraints in 2017 "related to the growing global demand for our vaccines and unexpected demand due to lack of competitive supply."

She said supply interruptions for the adult formulation of RecombivaxHB began in the first quarter of 2017 and that Merck "does not expect to be distributing the vaccine in the United States between now and the end of 2018."

A notice by the U.S. Centers for Disease Control posted last month said that GSK has sufficient supplies of adult and pediatric hepatitis B vaccines to address the anticipated gap in Merck’s supply.

While GSK has been able to cover for shortages in the past, a spokeswoman said the company is currently dealing with manufacturing issues of its own.

“GSK has a global shortage of hepatitis A, hepatitis B and combination hepatitis A and B vaccines. This is due to reduced manufacturing capacity for hepatitis A and B antigens, which we are working hard to return to normal supply,” the company said in an emailed statement.

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