The trial starts this afternoon, when several hundred people are expected to queue up for immunizations at the headquarters of the Boston Public Health Commission. Each of them will get a bracelet printed with a unique identifier code. Information about the vaccine's recipients, and the shot, will be entered into handheld devices similar to those used by delivery truck drivers.

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.