It’s been almost a month since a case of avian influenza was detected in poultry in the central United States. So it might seem that the epidemic—which over several months caused the destruction of 49.5 million chickens and turkeys—can safely be considered over.

But in fact, it may have only taken a break. And if it returns, as some experts predict it will, what one government official calls “the largest animal-health emergency in this country’s history” may turn out to be just an opening act. At risk, the next time, will be not just the egg and turkey farms of the Midwest, but the billions of birds being raised in the poultry-producing centers on the east and west coasts—effectively, most of the poultry economy of the United States.

And attempts to prevent it, by developing a vaccine, may paradoxically turn out to be almost as devastating—because international trading partners say they will bar imports of any birds vaccinated against the disease.

The dire uncertainty that surrounds bird flu emerged last week at a hearing held by the Senate Committee on Agriculture to examine how the epidemic unfolded and why it spread. (Here’s my earlier post from when the epidemic was still going on.) Just the numbers are jaw-dropping: To stop the movement of the disease, Dr. John Clifford, deputy administrator of the federal Animal Plant Health Inspection Service, testified that 42 million chickens and 7.5 million turkeys were killed. Those numbers equal 10 percent of the egg-laying hens in the entire United States, and 3 percent of the turkeys.

The cost of those lost birds, according to economist Thomas Elam of the Indiana-based consulting group FarmEcon, was $1.57 billion—but the further costs to businesses that support farms, to egg and poultry wholesalers, and to food service firms, pushed the loss to $3.3 billion. In addition, Clifford said, the US Department of Agriculture committed $500 million to emergency efforts to block the disease, and paid out $190 million to farmers whose birds were destroyed.

Brad Moline, a third-generation turkey farmer from Manson, Iowa, used the impact of the epidemic on his family’s farm as an example of what turkey growers face from avian flu. The Molines were forced to destroy their entire flock, 56,000 turkeys housed in 12 barns, wiping out at least two-thirds of their income for the year. Once a flock is destroyed, the birds have to be composted and the barns disinfected, and farmers cannot restock with baby birds (“poults,” if they are turkeys) until they get the all-clear. That’s if they can find birds to grow at all: Unlike poultry producers, turkey farms grow just one crop per year, and the hatcheries that supply the poults aim to hatch them only when they are needed—which was much earlier in the year than now.  “If we are lucky, we will be able to salvage this year with one flock, which we hope to repopulate sometime around August 1,” he told the committee.

As I reported earlier, animal-health authorities suspected—and virus analysis is now confirming—that while the flu was originally brought to the US by wild birds migrating down from Canada, most of the spread within the US was due to people and vehicles inadvertently carrying the virus from farm to farm. Rob Knecht, president of the Michigan Allied Poultry Industries, said that preventing that from happening again if the virus returns will be complicated and costly. Some examples he ticked off: requiring farms to book the episodic crews they use for short but labor-intensive tasks such as moving large flocks from barn to barn, and not sharing them with other farms. Creating worker “locker rooms” at local hotels, with clothing and boots provided, and driving them back and forth in disinfected vans. Constructing new shower facilities on farms, for every worker to use, or procuring mobile shower units that are delivered to farms by truck. Building disinfecting troughs and tire sprayers at every farm entrance and assigning workers to monitor the gates so that every vehicle goes through disinfection. “The financial investment on these changes,” he said, “can be very high.”

Producers are looking seriously at making those investments, though, because federal authorities are predicting the flu will come back. “There is the very real possibility of another major outbreak later this year,” Elam said. Clifford expanded on that: “USDA is treating the threat of more infections in the fall with the utmost seriousness,” he said. “It’s very likely that wild birds will carry the virus with them when they begin migrating south in the fall.”

The major chicken-producing states—Georgia, and the Delaware-Maryland-Virginia peninsula—should be particularly watchful, he added: “Although states in the Atlantic flyway have not been affected by this outbreak, it’s important that our state and industry partners begin preparations should the disease occur there.” That’s true for the Pacific Flyway as well; the first 2014 cases occurred there.

Possibly the most dismaying thing is that—unlike human flu—there’s no vaccine available, and while a USDA research unit is crashing research on a formula, using it could end up losing producers just as much money as letting the flu rampage. Currently, because of the outbreak, 18 countries have blocked imports of any American poultry or eggs, and another 38 countries have blocked imports from the states that were affected. But because of suspicions of the vaccine—which can suppress birds’ symptoms while still allowing them to pass the virus along—many countries won’t accept vaccinated birds for import.

“USDA believes that if a vaccine were used, some additional trading partners would ban all U.S. exports of poultry and eggs and not necessarily just those from the states currently affected by HPAI,” Clifford testified. “The loss of these markets could cost U.S. producers at least $3 billion in trade revenue.”

With a vaccine off the table, producers may have no option but to guard their farms’ borders, watch for new outbreaks, and wait. They may not have to wait long. Last week, the Minnesota Department of Natural Resources announced it had found a similar strain of virus in a chickadee, its second find in a wild bird so far—and the fall migration season hasn’t even begun.