This alerts people to potentially dangerous ingredients, but not all allergens are on that list. In addition, companies sometimes need to warn consumers about possible “cross-contact” with allergens, telling them that something “may contain” peanuts, which can create more confusion.

Alicia M. Ames, of Elbridge, N.Y., said her 4-year-old son, Jackson, is allergic to sesame, eggs, peanuts and legumes. Sesame is not part of the Food and Drug Administration’s labeling law, and its presence is sometimes hidden under obscure descriptions like “natural flavors” or “spices.”

More than one million children and adults are estimated to have a sesame allergy, and the F.D.A. is considering adding it to the list of allergens that manufacturers must include on packages.

Ms. Ames bakes her own bread, but her supplies of safe flour and yeast are running low. “Our worry is that these foods aren’t going to be available, and what are we going to feed our family?” said Ms. Ames, 32, a musician.

Her unease is shared by others across the country.

Recently, Elana D. Zimmerman put on gloves and a mask and ventured out to many grocery stores in her neighborhood on the Upper East Side of Manhattan. She did it again the next day. And the day after that. Ms. Zimmerman, 36, has a 1-year-old daughter and a 2-year-old son, both with severe allergies.

Laura C. Schorn, of Aurora, Colo., has been going to stores at various times in the day, hoping to catch a lucky break and arrive after a restock. Ms. Schorn, who has an intolerance to wheat and soy, said she has left stores crying, feeling defeated.

“My fear right now is less that I’m going to get the virus and more that if I do get it and I become quarantined, I’m not going to have enough food to get through it,” said Ms. Schorn, 25, who works as a supervisor at a restaurant chain.