This may be because the volunteers were already immunologically tolerant to the potato antigens. Potatoes have two good characteristics they are easy to transform and they are not usually propagated from seeds. They are planted as pieces of tuber containing an eye . This means that recombinant genes introduced into potatoes are not likely to get loose and get into other potatoes. For this reason, work is now being done on transgenic bananas.
Bananas have their own problems.
The researchers also don't think local villagers will grow these transgenic vaccine bananas outside their hut. They will likely be grown in greenhouses in each country where trained workers monitor the genetics and growth of the plants and other trained people administer controlled amounts of the banana vaccine. The most likely success story for edible vaccines will be with viral capsid proteins that aggregate into particulate structures. These particles are much better immunogens than soluble proteins and may not require an adjuvant like cholera toxin. Encouragingly, research has shown that some plant viruses cause the production of antibodies when they are fed to mice.