Authorities were also told to alert the public and urge anyone experiencing symptoms after eating at the Ba Duoc sandwich shop in Cam Thanh Ward to seek medical attention.
Investigators have been asked to trace the origins of ingredients used at the shop, identify potentially contaminated supplies and conduct laboratory tests.
The department called for strict penalties if food safety violations are confirmed, with investigation findings to be made public.
The first 16 patients were hospitalised on the evening of October 9. By October 10, the number had risen to 55.
Patients reported abdominal pain, fever and vomiting after eating sandwiches purchased from the Ba Duoc shop on Duy Tan Street.
Health inspectors and police subsequently inspected the premises.
Initial checks found that the owner had not provided documentation establishing the origins of ingredients used to prepare the sandwiches.
Authorities ordered the shop to suspend operations temporarily and collected food samples for testing to determine the cause of the suspected poisoning.
