First, some numbers to compare to your number of 685U/ml:
from a lab - see the chart at the link, you appear to rank #2 or #3 of 6 protective antibody levels. (#1 is wildly high) Also, to compare:
“From the data that Testing For All users...Typical result following natural infection ~ 0.8 – 250 U/mL based on analysis of 72 samples.”
https://www.testingforall.org/understanding-your-roche-anti-sars-cov-2-s-test-result/
“Based on the literature and on his own research, Landau said natural infection with wild-type virus generally confers a titer of 1:400. With Delta, that gets bumped down to 1:100 — which is still pretty good, he noted.
Immunity conferred by the mRNA vaccines, on the other hand, typically lands at a titer of 1:1,000 when looking at the original virus, he added.”titers for the Johnson & Johnson vaccine, at this point, don’t appear to be as high as the mRNA products”
https://www.medpagetoday.com/special-reports/exclusives/95156
Your JJ vaccine is doing exactly what it was said to do - strengthen over time and good to go for 8 months against original strains, and holding its own against Delta hospitalization and death.. It’s effective against beta, alpha and delta, but how do we know about other variants yet? At 75, it’s your risk assessment as to whether you need a JJ booster. Also bear in mind even antibody testing labs say, antibody testing is not definitive, and T-cell involvement (the killer cells that command the B cells) is integral to overall antibody effectiveness.
Research in the US and in Amsterdam:
“Current data for the eight months studied so far show that the single-shot Johnson & Johnson COVID-19 vaccine generates a strong neutralizing antibody response that does not wane; rather, we observe an improvement over time,” Dr. Mathai Mammen, head of research and development at J&J’s Janssen vaccine arm, said in a statement.
The company said one dose of the vaccine elicits both a lasting antibody response and generates immune cells called T-cells that last eight months, also.
https://www.cnn.com/2021/07/01/health/johnson-vaccine-delta-variant/index.html
Thank you for your very thoughtful response and providing information I did not know.