Question: What are the indications that one (age 65+) is ready for a booster shot, based on the antibodies that show up in the blood work, having had the second vaccine seven months ago?

Answer: Per CDC guidelines, “Antibody testing is currently not recommended to assess for immunity to SARS-CoV-2 following COVID-19 vaccination.”  Studies are underway to understand what threshold of antibodies is required to stave off infection/reinfection.  Those data, however, are not yet available and we are still uncertain to what degree and for how long people with detectable antibodies are protected.  In the meantime, data have been accumulating that show vaccine-derived protection against infection is waning over time and in response to the Delta variant.  These declines have been more substantial in older adults (ages 65+) and the risk/benefit balance for receiving a booster is the most favorable for adults ages 65+ (per ACIP).  For these reasons, I suggest that you follow the simpler guidelines for booster shots (Table), and I do not suggest using antibody test results to make decisions about whether to receive the booster.  As always, please talk with your healthcare provider.  

Table. Booster Eligibility (from CDC)