The quick detection of an ultra-rare blood clotting reaction in some Covid-19 vaccine recipients showed the power of a federal warning system for vaccine safety issues, but experts worry that blind spots in the program could hamper detection of other unexpected side effects.

Before the pandemic began, the Food and Drug Administration had scaled back a program it used successfully to track adverse events during and after the 2009 H1N1 influenza pandemic, and the agency is still ramping up its replacement, said Dr. Robert Chen, scientific director of the Brighton Collaboration, a nonprofit global vaccine safety network.

“It’s purely bad luck they were in between systems when Covid hit,” said Chen, who helped create the existing U.S. vaccine safety systems during nearly 30 years at the Centers for Disease Control and Prevention.

FDA officials acknowledged that some data analysis won’t start for weeks or months, but said the government is watching for vaccine reactions with “state-of-the-art” systems.

“FDA and CDC have robust safety and effectiveness surveillance systems in place to monitor Covid-19 vaccines authorized for emergency use in the U.S.,” Abby Capobianco, an FDA spokesperson, wrote in an email.