The last reported US illness was on October 31, while the most recent illness in Canada was early this month.
Public health officials are advising people to throw out any romaine lettuce they may have.
Nonetheless, the major produce companies headquartered in the Salinas Valley have largely shifted their production to the Yuma region, which began shipping romaine a couple of weeks ago, Horsfall said.
He suspects the USA has gone a step further than Canada in part because US authorities reported 32 cases of E. coli, 13 of which involved a person who was hospitalized.
The Canadian Food Inspection Agency has not yet issued a recall on the romaine lettuce because the source of the outbreak hasn't been identified. Further, anyone who develops symptoms of e.coli infection are advised to talk to a healthcare provider, to report the illness to the health department, and to record the foods consumed in the week prior to the onset of the illness.
They should also wash any fridge drawers, containers or countertops where lettuce may have been stored with soap and warm water. In the earlier outbreak, the warnings about romaine from Yuma might have been confusing, he said.
The Head of the U.S. Food and Drug Administration says the source of the romaine lettuce outbreak may be in California; however investigations are still ongoing.
Romaine lettuce grown in South Korea is usually used in salads at high-end restaurants. In the US, 199 people were infected and three died, according to the CDC. Ill people range in age from 7 to 84 years, with a median age of 24.
This is at least the third outbreak of E. coli linked to leafy greens in the United States and Canada in the last two years.
After laboratory analysis, the genetics of this illness are reported to be linked to a previous E. coli outbreak from December 2017.
Redfield said that authorities are continuing to investigate, and emphasized the need for consumers to throw out all Romaine lettuce, saying there are "no exceptions".
"Most E. coli strains are harmless to humans, but some varieties cause illness".