Where were these harmful batches administered? When there is some kind of contaminated food discovered, the location of its distribution is given nationwide attention.
Another article points that the distribution of good batches was focused on some area, but the bad ones suspiciously were sent to the 4 corners of the country, spread out, avoiding cluster effects. Check out Diana West
https://dianawest.net/Home/tabid/36/EntryId/4390/Gone-with-the-Flu-Other-Horror-Stories.aspx