From the Guidelines

A Hepatitis B (Hep B) surface antibody titer of 90 signifies immunity to Hepatitis B.

A titer of ≥10 mIU/mL is generally considered seroprotective and indicates immunity to Hepatitis B, as stated in the study by 1.
The presence of anti-HBs is generally indicative of immunity to HBV infection 1.
Anti-HBs levels of ≥10 mIU/mL are considered seroprotective; however, different assays have different assay cutoff values based on which reported levels of anti-HBs might vary depending on the assay used 1.

Immune individuals have long-term protection and do not need further periodic testing to assess anti-HBs levels, unless they are immunocompromised 1.
Immunocompromised persons might need annual testing to assess anti-HBs concentrations 1.
Vaccine recipients are differentiated from patients with resolved infection by the detection of anti-HBs without total anti-HBc 1.

A Hepatitis B surface antibody titer of 90 is well above the protective threshold of 10 mIU/mL, indicating strong immunity to Hepatitis B.
This individual is considered immune and does not require further vaccination or testing, unless they are immunocompromised or have other risk factors 1.

From the Research

A Hepatitis B (Hep B) surface antibody titer of 90 signifies a level of immunity to Hepatitis B virus (HBV) infection.

The titer level is measured in milli-international units per milliliter (mIU/mL) and indicates the presence of antibodies against the HBV surface antigen (HBsAg) in the blood.
According to a study published in 2019 2, a titer of >100 mIU/mL is considered a sufficient response to HBV vaccination, and most subjects with this level of titer maintained an anti-HBs titer of >10 mIU/mL at 1 and 2 years after vaccination.
However, the significance of a titer of 90 mIU/mL is not explicitly stated in the provided studies, but it can be inferred that it falls within the range of a sufficient response to HBV vaccination.

A study published in 2016 3 suggests that a titer of anti-HBs ≥ 10 IU/L is considered seroprotective, but it does not provide 100% protection against infection and clinically manifest illness.
Another study published in 2022 4 found that the optimal cutoff value to predict a decreasing population in HBsAb level was 8.53 mIU/mL, with a sensitivity and specificity of 94% and 70%, respectively.
It is essential to note that the interpretation of anti-HBs titers can be complex and may depend on various factors, including the individual's immune status and medical history.

The provided studies suggest that regular serological testing and booster vaccination may not be necessary for immunocompetent individuals with anti-HBs ≤ 10 IU/L 3.
However, individuals with a previous HBV history, those taking antiviral drugs, and/or those with low levels of HBsAb may require reimmunization with the hepatitis B vaccine in a timely manner to maintain optimal immunity against HBV infection 4.
The use of ELISpot assay to study HBs-specific B cell responses in vaccinated and HBV infected humans may provide valuable information for designing novel therapeutic strategies against CHB 5.