Public Health England said it is "fairly confident" there was "a single incident that happened on a single day to a limited number of products" following the blood poisoning death of a baby and the infection of 14 others.
Dr Deborah Turbitt told Good Morning Britain: "This company has made products for years and years and years and there has not been a single incident like this.
"It is a very unusual incident and we haven't seen any other babies become ill.
"We think that the number of babies who have been infected we now know about and can be fairly confident that there aren't other babies and it is not an ongoing problem."
Questions have been raised over how a batch of intravenous fluid could potentially have become contaminated, after the death of one baby.
The suspected contaminated batch of a food supplement linked to the blood poisoning of 17 babies, was sent to 22 hospitals.