Vaccination initially elicits an IgM response, followed by an IgG response that builds over the following weeks.
IgM accounts for about 5-10% of all antibodies, while IgG is the most common antibody accounting for about 75-80% of total antibodies.
An infant's ability to mount its own IgG response during the first year of life is much lower compared to that of an adult, and the IgM response isn't fully functioning until approximately 1-3 years of age.
This maturation of the infant immune system renders the birth, 2, 4, and 6-month vaccines ineffective for many babies.
All the risk, none of the benefit.
And that's assuming you believe antibody response to vaccination is beneficial.
"For infants younger than 4 months of age, those weighing less than 2500 g at birth had lower IgG levels than those weighing more than 2500 g. Infants younger than 1 month whose gestational age had been less than 36 weeks had lower IgG levels than those greater than 36 weeks. Infants aged 1 to 4 months whose gestational age had been less than 36 weeks had lower IgG levels than did those 40 or more weeks."
"The infants who had the highest IgG levels at birth showed a significantly more rapid decrease in their IgG concentrations during the first 6 weeks of life than those with the lowest initial IgG levels. This finding may possibly indicate a later start of their own synthesis of IgG in infants who have a high serum concentration of maternal IgG at birth. The infants exhibited a very early capacity for IgM synthesis and the mean IgM curve rose steeply during the first two weeks of life, subsequently becoming flatter. The infants with recurrent infections of different kinds showed, on the average, twice as high IgM concentrations at the age of 1 year as the children in the previously reported BWC series."
"Maternal immunoglobulin G (IgG) is transported across the placenta, mainly in the third trimester. Because of this, babies born prematurely have less IgG.
After birth, the maternally derived IgG levels fall to a physiological nadir of 400 mg/dL at 3–6 months, at the same time, the infant’s production of IgG is not fully developed. Preterm babies have a more pronounced nadir. At the age of 1 year, IgA levels are approximately 20% of adult levels, IgG approximately 60% of adult levels and IgM approximately 75% of adult levels"