Complex sentence: The vaccine is not effective in children under the age of two; children under the age of two would normally have all their food regulated by their parents and should therefore be expected to be at low risk of exposure to typhoid.
Split into simpler sentences: The vaccine is not effective in children under the age of two. Normally, children under the age of two would have all their food regulated by their parents and should therefore be expected to be at a low risk of exposure to typhoid.