When people arrive for their shots, they will get an ID bracelet with a barcode. Next, basic information - name, age, gender, address - will be entered into the patient tracking database. There will be electronic records, too, of who gave the vaccine and whether it was injected into the right arm or the left, and time-stamped for that day.

The resulting trove of data could be used to figure out why some patients had to wait longer than others to be vaccinated. "When all is said and done," said Jun Davantes, director of product management at EMSystems, the company that makes the technology, "Boston will be able to identify where there are certain bottlenecks in the process and hopefully improve it the next time around."

Ultimately, city health authorities said, they envision creating a network across the city that would allow public and private providers of flu shots to add data to a registry.

Infectious disease specialists in Boston and elsewhere predicted that the registry approach could prove even more useful if something more sinister strikes: a bioterrorism attack or the long-feared arrival of a global flu epidemic. In such crises, the registry could be used to track who received a special vaccine or antidote to a deadly germ.