It’s different than that. Sustained “neutralizing antibodies” is needed for sustained protection from the virus. The antibodies testing is a way to measure the ability of a vaccine to have a long lasting effect. The Moderna and Pfizer vaccines have been shown from the beginning to have a short term effect. New peer review for Moderna:

Abstract
SARS-CoV-2 mutations may diminish vaccine-induced protective immune responses, particularly as antibody titers wane over time. Here, we assess the impact of SARS-CoV-2 variants B.1.1.7 (Alpha), B.1.351 (Beta), P.1 (Gamma), B.1.429 (Epsilon), B.1.526 (Iota), and B.1.617.2 (Delta) on binding, neutralizing, and ACE2-competing antibodies elicited by the vaccine mRNA-1273 over seven months. Cross-reactive neutralizing responses were rare after a single dose. At the peak of response to the second vaccine dose, all individuals had responses to all variants. Binding and functional antibodies against variants persisted in most subjects, albeit at low levels, for 6-months after the primary series of the mRNA-1273 vaccine. Across all assays, B.1.351 had the lowest antibody recognition. These data complement ongoing studies to inform the potential need for additional boost vaccinations.

https://science.sciencemag.org/content/early/...ce.abj4176

Also, this may be of interest:

Antibodies fight off the new coronavirus, but what do T cells do?

https://www.medicalnewstoday.com/articles/ant...t-cells-do