A baby-formula shortage has been going on even before the shutdown of a Michigan factory made it worse.

The grocery data analyst Datasembly says there was an uptick in stores running out of formula last July, but the shortage has accelerated since November. That’s three months before a recall prompted the shutdown of a Similac plant in Michigan. As of the end of April, the company says 43-percent of stores were out of formula.

Notre Dame Mendoza College of Business Professor Kaitlin Wowak says while pandemic-era shortages of items like masks and sanitizer reflected products being made overseas and imported, most formula for the U-S market is made domestically. But Wowak says labor shortages and supply chain issues have slowed production and stretched how long it takes for trucks to deliver products to store shelves. And only a few companies make formula — the two leading brands, Similac and Enfamil, account for two-thirds of the market.

Abbott Laboratories manufactures Similac, Alimentum, and Elecare at its plant in Sturgis, Michigan, three miles from the Indiana line. The plant suspended operations in February after two babies died and two others were sickened by a rare bacterial infection. A subsequent F-D-A inspection found safety and sanitary violations at the plant, though Abbott has said company and F-D-A tests concluded it’s unlikely the formula was the source of the infections.

The White House has said it’s finalizing plans to import formula to plug the gap. Wowak says the move can’t hurt, but says foreign plants may not have extra inventory to spare any more than American factories do.

And unlike other shortages, Wowak says the formula shortage is potentially life-threatening because it’s a product without an available substitute.