The HepB vaccine given to a newborn immediately after birth is capable of harm.
The Engerix-B HepB vaccine contains 0.71 mg/ml NaH2PO4.
The pediatric dose is 0.5 ml. A newborn male in the 50th percentile weighs 3.6 kg.
Unit conversion yields
This amount is similar in magnitude to what caused a harmful effect seen when several infants were accidentally exposed to intravenous NaH2PO4.
In those cases the infants were exposed to between 4.2 mEq and 14.3 mEq per kg of body weight.
1 mMol NaH2PO4 => 1.5 mEq Na+
119.977 g/mol = molecular weight of NaH2PO4
then unit conversion yields
NaH2PO4 and Na2HPO4 are supposed to be balanced in the vaccine initially, but it is not clear what happens when they mix with the infant's fluids and tissues.
The above calculations show that an effect is plausible.
Similarly, this study demonstrated the possibility of exceeding the buffering capacity of low birth-weight infants exposed to 30 ppm KH2PO4 continuously for 4 days.
That exposure was two orders of magnitude higher than the examples above, however the study did not describe the point at which negative effects actually began to occur.
The toxic burdens were accumulating all the way up to the point when study samples were taken.
The first equation above also illustrates one benefit of delaying vaccination.
An older child weighs more. The additional weight results in a lower concentration of exposure.