Two people in each group received a dummy vaccine, or placebo, while the rest got one of three levels of the TA-CD vaccine.
(Two people in each group; received; a dummy vaccine, or placebo; T:while the rest got one of three levels of the TA-CD vaccine) (the rest; got; one of three levels of the TA-CD vaccine)
Two people in each group the rest
received got
a dummy vaccine, or placebo T:while the rest got one of three levels of the TA-CD vaccine
one of three levels of the TA-CD vaccine