If the first coronavirus vaccines arrive as expected in the coming year, government researchers will face a monumental challenge: monitoring the health of hundreds of millions of Americans to ensure the vaccines don’t cause harm.

Purely by chance, thousands of vaccinated people will have heart attacks, strokes and other illnesses shortly after the injections. Sorting out whether the vaccines had anything to do with their ailments will be a thorny problem, requiring a vast, coordinated effort by state and federal agencies, hospitals, drugmakers and insurers to discern patterns in a flood of data. Findings will need to be clearly communicated to a distrustful public swamped with disinformation.

For now, Operation Warp Speed, created by the Trump administration to spearhead development of coronavirus vaccines and treatments, is focused on getting vaccines through clinical trials in record time and manufacturing them quickly.

The next job will be to monitor the safety of vaccines once they’re in widespread use. But the administration last year quietly disbanded the office with the expertise for this job. Its elimination has left that long-term safety effort for coronavirus vaccines fragmented among federal agencies, with no central leadership, experts say.

“We’re behind the eight ball,” said Daniel Salmon, who served as the director of vaccine safety in that office from 2007 to 2012, overseeing coordination during the H1N1 flu pandemic in 2009. “We don’t even know who’s in charge.”

A Health and Human Services spokesperson declined to answer questions why the vaccine office, set up in 1987, was closed or how the health agencies were planning to track the safety of vaccines once they are injected into millions of people. In a brief statement, she said Operation Warp Speed was working closely with the Centers for Disease Control and Prevention “to synchronize the IT systems” involved in monitoring vaccine safety data.

Scientists at the CDC and the Food and Drug Administration have decades of experience tracking the long-term safety of vaccines. They’ve created powerful computer programs that can analyze large databases.

“It’s like satellites looking at the weather,” said Dr. Bruce Gellin, president of the Sabin Vaccine Institute, who headed the National Vaccine Program Office from 2002 to 2017.

But monitoring hundreds of millions of Americans who may get different coronavirus vaccines from a variety of drugmakers by summer is like tracking a major storm beyond anything researchers have dealt with before.

In the 1970s, the U.S. government set up large-scale programs to monitor vaccine safety. There was a system for parents to report symptoms their children experienced after getting a vaccine.

In 1990, the CDC set up a new way to track vaccines. The agency worked with health care organizations to get updates on people’s medical conditions. That system covers 12 million people.

When the H1N1 flu hit in 2009, Salmon recognized that these methods didn’t track enough people to quickly pick up rare symptoms. He reached out to researchers at Harvard to build a new system, which came to be known as PRISM. Ten states supplied vaccination records, and five health insurance companies shared anonymous information about 38 million members. PRISM then connected the two databases to track insurance claims in the wake of vaccination. “That really gave us a ton of data,” Salmon said.

The researchers could come up with a background rate of a host of medical conditions. If the H1N1 vaccine was linked to cases that matched the background rate, they could dismiss the symptoms as ordinary. Only if they rose above the background rate would they be considered unusual and warrant a closer look.

Scientists from various federal agencies gathered every two weeks to share data and look for worrying clusters of symptoms. Every month, outside experts reviewed the evidence and released public reports. “Vaccine programs are contingent on trust,” Gellin said, “and transparency is a huge element of that.”

As emerging viruses caused outbreaks of Ebola, Middle East respiratory syndrome and other diseases, experts called for more preparations for the next pandemic. In 2016, President Barack Obama set up a global health security office at the National Security Council. But in 2018, the Trump administration disbanded that office, saying it was streamlining bureaucratic bloat.

The next year, the National Vaccine Program Office met a similar fate. Alex Azar, secretary of health and human services, said in a letter to Sen. Patty Murray, D-Wash. and the ranking member of a health subcommittee, that the merger, as part of a broader department reorganization, would “increase operational efficiencies by eliminating program redundancies and decreasing program costs.”

Dr. Nicole Lurie, who was assistant secretary for preparedness and response at HHS during the 2009 pandemic, said the loss of the vaccine safety office was especially costly once the coronavirus pandemic hit. “The coordinated leadership for stuff like this would likely come from the National Vaccine Program Office,” she said.

An FDA spokesperson said that in the absence of the National Vaccine Program Office, FDA and CDC staff members were relying on relationships they had built across the agencies, meeting regularly to discuss their separate projects.

That leaderless effort concerns Lurie. “There’s no sort of active coordination to bring all the information together,” she said.