Lots of Merck vaccine news today, only this isn’t good news for the drugmaker, which is experiencing problems with obtaining a key ingredient for its Zostavax shingles vaccine, Dow Jones reports. And the delays could extend up to six weeks, according to a Merck spokeswoman.

“We are continuing to accept orders for Zostavax,” she tells the Dow. “However, customers will experience shipping delays in the coming months. We will work to fill orders as product becomes available. We, of course, will continue to monitor closely the supply and demand for Zostavax and expect to provide an update to customers in coming weeks.”

The problem began last year, when Merck made a batch of bulk varicella that was less potent than required. Bulk varicella is used for Zostavax and its Varivax chickenpox vaccine for children, so Merck stopped filling orders for a combo vaccine called ProQuad. Manufacturing later resumed, but the FDA must clear process improvements before wider use of the ingredient can start again, Dow writes.

Meanwhile, Merck has given Varivax priority over Zostavax for use of the bulk ingredient, according to Merck. Why? Varivax sales more than doubled last year to $855 million, the Dow notes, after a national vaccine program recommended a second dose. Zostavax sales were $236 million last year.