Antibody levels decrease after two doses of COVID-19 vaccine - study

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Immunoglobulin G (IgG) is the most common type of antibody circulating in the human body. It represents approximately 75% of the serum antibodies.

IgG is secreted as a monomer that is small in size allowing it to easily perfuse tissues. It is the only antibody isotype that has receptors to facilitate passage through the human placenta, thereby providing protection to the fetus in utero. Along with IgA secreted in the breast milk, residual IgG absorbed through the placenta provides the neonate with humoral immunity before its own immune system develops. Colostrum contains a high percentage of IgG, especially bovine colostrum. In individuals with prior immunity to a pathogen, IgG appears about 24–48 hours after antigenic stimulation.
Therefore, in the first six months of life, the newborn has the same antibodies as the mother and the child can defend itself against all the pathogens that the mother encountered in her life (even if only through vaccination) until these antibodies are degraded. This repertoire of immunoglobulins is crucial for the newborns who are very sensitive to infections, especially within the respiratory and digestive systems.
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When a person is vaccinated with a mRNA vaccine, within a few hours the Immunoglobulin_G (IgG) and the Neutralizing Antibody (NAb) appear rapidly. I have previously asserted that using the Israel data, that the protection from transmissibility the mRNA vaccines provide drops by about 15 percent per month. This paper Waning Immune Humoral Response to BNT162b2 Covid-19 Vaccine over 6 Months validates this observation:

Six months after receipt of the second dose of the BNT162b2 vaccine, humoral response was substantially decreased, especially among men, among persons 65 years of age or older, and among persons with immunosuppression.

In other words it suggests that booster shots should be scheduled every couple of months if you have received this type of vaccine.