British pharmaceutical giant GlaxoSmithKline (GSK) is facing mounting questions over a global shortage of its hepatitis B virus (HBV) vaccine, which has led to rationing of the shots across the United Kingdom. So far, the company’s supplies to the larger U.S. market appear to be unaffected, The Sunday Telegraph reports.

Earlier this month, Public Health England (PHE) advised doctors across the country to limit prescription of the vaccine to only those at greatest risk of infection, after Merck, another big drugmaker, and GSK cited manufacturing issues earlier this summer. However, in July, the U.S. Centers for Disease Control and Prevention posted a notice saying that GSK had sufficient supplies of both adult and pediatric hepatitis B shots to address the anticipated gap in supplies. 

The disparity has led to suggestions from U.K. liver disease advocates that GSK may be “prioritizing” the more lucrative American market over its home country and that shortages of the vaccine may not be as widespread as suggested. Recently, the World Health Organization, which is responsible for monitoring stocks of vaccines around the world, said there is no global shortage of the hepatitis B vaccine. 

In response to the allegations, a spokesman for GSK has doubled down on the company’s claim, saying its HBV vaccine is in fact limited. However, the company did not comment on the apparent disparity in supplies between the United States and the United Kingdom. PHE has also told reporters that it has no evidence to suggest such favoritism.

Meanwhile, British health officials say hepatitis B rationing measures are expected to remain in place until early next year. GSK is currently working with the Department of Health to ensure that there is enough vaccine to inoculate adults and children at high risk of infection. However, advocates remain confused about the “global shortage.”