Generally speaking, the settings were shortly after induction and took place at boot camps
, basic training depots and the like. There was no need for
vaccination while 'in-country' since the process was to be adequately vaccinated before departure.
To possibly connect, or disconnect, some dots...It does not make sense that Viet Vets were the infecting source through the air/jets. You see, the Vets would not be in the innoculation line with the newly-minted recruits.