Editor’s note: This story is an update of an article originally published on July 22.

The U.S. Centers for Disease Control and Prevention (CDC) has released new information about a major listeria outbreak linked to deli meat processed at a Boar’s Head plant in Virginia.

The median age of people sickened in the outbreak is 77, the CDC noted.

The agency also exhorted Americans to check their refrigerators, discard any potentially contaminated deli meat, and sanitize any surfaces or objects that may have been in contact with those products.

Some of the deli meat may have “sell-by” dates into October 2024, the CDC noted.

Boar’s Head Recall Dates Back to July

In July, Boar’s Head recalled all its liverwurst sold ready to slice at retail deli counters because of concerns about potential contamination with listeria, a type of bacteria that can cause serious and even life-threatening infection in vulnerable people.

The USDA website offers specific information about foods included in the recall, including types of Boar’s Head deli meats and “sell by” dates on items shipped to retailers. The USDA is urging customers to throw away any recalled deli meats or return them to their place of purchase, and to clean their refrigerators thoroughly to prevent any possible cross-contamination.

Reports Likely Undercount the Number of Sick People

“The true number of sick people in this outbreak is likely higher than the number reported, and the outbreak may not be limited to the states with known illnesses,” the CDC said.

“This is because some people recover without medical care and are not tested for listeria,” the CDC said on August 27. “In addition, recent illnesses may not yet be reported as it usually takes three to four weeks to determine if a sick person is part of an outbreak.”

Illnesses have occurred over several months, with the first case dating back to late May, the CDC said.

It can take up to 10 weeks for some people to have symptoms of listeriosis after eating contaminated foods, the CDC noted.

CDC Connected the Outbreak to Sliced Deli Meat

All but 3 of the 44 individuals that public health officials interviewed as part of the investigation into the listeria outbreak as of August 27 reported eating sliced deli meats at some point in the month before they got sick, the CDC said. Investigators had not identified any cases of people becoming sick after eating prepackaged deli meats.

Among those interviewed as part of the investigation, most people recalled eating turkey or liverwurst sliced at a deli, and a handful of people also recalled consuming ham, the CDC said. They all got meat from different types of grocery store deli counters, according to the CDC. But they all appeared to be infected with a similar strain of bacteria.

The CDC noted that listeria spreads easily from deli equipment, surfaces, hands, and food. Refrigeration doesn’t kill listeria, the CDC said. But cooking deli meat at high heat can kill any germs that may contaminate these products.

For this reason, the CDC recommended heating deli meat to 165 degrees F or until steaming hot, then letting it cool before consumption if a cold sandwich is preferred.

“It is not surprising to find listeria contamination of deli meats,” says Nancy Freitag, PhD, a professor and the head of pharmaceutical sciences at the college of pharmacy at the University of Illinois in Chicago. “The sources of contamination can vary, but often come from food-processing facilities. Listeria has long been associated with deli meats and soft cheeses; but more recent contamination has been found with a variety of foods, including ice cream and produce.”

She adds, “The best way to avoid listeria is to cook your food, which unfortunately is not always practical” — as with ice cream, for instance.

Symptoms of Listeria Infection Can Range From Mild to Severe

Most people who become infected experience only brief, mild illness, but people with autoimmune conditions and those older than 65 can develop deadly infections in the bloodstream or brain. Pregnant people may experience some flu-like symptoms, but are also at risk of miscarriage, stillbirth, or premature delivery, as well as life-threatening infections in their newborns.