The most important trade war to come may have nothing to do with cars, steel, or soybeans. It may involve a virus.

The vast agricultural enterprise in southern China is the source of most new flu strains. Under rules established by the World Health Organization in 2011, China has routinely shared samples of them with researchers in the United States and other countries where vaccines are developed. But recent trade tensions may be leading it to change course.

Influenza is a serious disease. The seasonal variety kills thousands of people worldwide each year. But its impact pales in comparison with pandemic flu. That is the kind that emerges every few years and spreads like wildfire around the globe, sometimes killing millions – as it did in 1918.

Public health officials fear that a new flu pandemic may be on its way. A strain of bird flu known as H7N9 originated among poultry in China in 2013 and evolved into a form that can infect humans. It reportedly kills 40 percent of those who become ill. It has not yet spread beyond China and is not yet contagious from human to human, with only about 1,600 reported cases so far. However, should it mutate into a form that can be spread between people, the threat could be worldwide.

A vaccine against H7N9 could save thousands of lives – maybe millions. However, China is refusing to share virus samples despite repeated requests. It has even refused to share clinical data on infected patients. Information flowed freely soon after the strain first emerged, but it has slowed to a stop. Researchers have obtained a few samples from Taiwan and Hong Kong, but those may not be enough to develop a vaccine.

China claims that it has almost eradicated the virus with a single poultry vaccination campaign, so further vaccine development is not needed. But public health experts are not reassured, since mutations are always possible.

This is not the first time China has been secretive about a pandemic threat. In 2002, it hid information on SARS, and in 2005, it hoarded samples of a previous bird flu strain, H5N1. But until recently, it had been cooperating with the World Health Organization's more recent rules on sharing flu strain samples.

Why the sudden secretiveness? One possibility is that China is trying to avoid harm to its poultry industry. Another is that it is looking for a head-start over the U.S. and other countries in developing a vaccine on its own.

However, there is also a strong possibility that it is responding to the brewing trade war with the U.S.

Among the Chinese exports on which the U.S. has threatened to impose tariffs are pharmaceutical products, including vaccines, and other medical supplies. Virus samples seem to be part of the mix of products over which we are negotiating, even though no one actually owns them, so tariffs should not apply. China may be waiting for the outcome of those talks before allowing virus samples out of the country.

While the U.S. and China trade tariff threats, the risk grows that we will be unprepared if a pandemic arrives. The fight over bird flu samples is a game of chicken in more ways than one, and it could have lethal consequences.

International trade is more than just an economic issue, as important as that is. It is also a matter of public health. Unless our trade policy recognizes that as a priority, a lot more may be at stake than the prices we pay for imported goods.

Robert I. Field is professor of law and public health at Drexel University and founder and editor of the Health Cents blog.