Covid-19 has rocked the U.S., infecting half a million people to date. While the global health crisis has led to widespread shortages of drugs and medical supplies, including masks and ventilators, it has also raised questions about where this country’s antibiotics are produced, and whether the pandemic might affect supplies.

Many Americans, for example, are prescribed amoxicillin — a penicillin based antibiotic — for bacterial infections, such as ear or sinus infections. If you look at it, the pill bottle may say the drug comes from Aurobindo, one of the largest Indian drug makers. But the drug is produced, sterilized, pressed, packaged and shipped by a complex and opaque network of global companies and trading firms that move it towards hospitals, clinics and retail pharmacies like CVS and Walgreens. The starting point, experts agree, is usually the same: by some estimates, 90 percent of the world’s supply of antibiotics originate at huge chemical and pharmaceutical factories in China.

China is the world’s biggest exporter of so-called “active pharmaceutical ingredients,” or APIs, in antibiotics. According to IHS Markit, China shipped $3.6 billion worth of antibiotics last year, accounting for 64 percent of global antibiotics exports by weight. The following chart shows some of the most widely used antibiotics. 

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Still, the data is murky. Estimates vary on how much of America’s antibiotics come from China, partly because of how the data is collected. The F.D.A., for example, reports that 13.4 percent of American drug imports came directly from China in 2018. But that figure does not seem to factor in the source of the raw materials — the APIs and precursor chemicals that China controls. India, for example, is the world’s leading exporter of generic drugs, but some studies report that the country gets about 80 percent of its APIs from China.

This may explain why a confidential U.S. Commerce Department study cited by the journalist Bob Woodward found that 97 percent of all antibiotics in the U.S. come from China. Another report, released last year by the U.S.–China Economic and Security Review Commission, a congressional commission tasked with monitoring relations with China, also highlighted U.S. dependence on Chinese made drugs. In a report to Congress, the committee said that it was “highly likely” that most generic drugs imported into the U.S. contain active ingredients sourced from China.

The F.D.A. approval is needed by domestic or overseas manufacturers seeking to sell goods in the US. But the F.D.A. does not have oversight over the sources of all raw material, nor does it require drug companies to list the countries of origin on products. 

China hasn’t always dominated antibiotics manufacturing. American companies were the first to mass produce penicillin, beginning in the 1940s, during World War II. By D-Day, the U.S. was manufacturing huge amounts of penicillin to treat wounded soldiers. Much of it was produced in Brooklyn, New York, at a manufacturing facility operated by the American drugmaker Pfizer.

In the 1980s and 1990s, however, China began to modernize its manufacturing operations. The country’s state-backed chemical and drug companies invested heavily in penicillin fermentation, helping drive down global prices. In 1988, the U.S. had at least 29 penicillin and antibiotics manufacturing facilities located around the country. Today, there is not a single large manufacturer of these critical drugs.

The last penicillin fermenter in the U.S. was a Bristol-Myers Squibb plant in East Syracuse, N.Y. The company announced plans to close the plant in 2004, leaving the U.S. dependent on imports. Today, China’s booming antibiotics manufacturing industry is concentrated in the northern and eastern provinces. Fermentation plants require cool temperatures to keep refrigeration costs down. The plants also produce a tremendous amount of air and water pollution, and China’s regulatory controls have long been lax. Only in recent years has the Chinese government begun to enforce rules on air pollutants and wastewater treatment.

Most Americans can’t know for sure where their antibiotics originated, but it’s likely they originated in north China, made their way by ship to India, and were packaged and sent, through a network of brokers and traders, to  hospitals, clinics and pharmacies in the U.S.

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David Barboza, Lynn Zhang and Kara Greenberg contributed research from Boston.