The goal of this clinical study is to learn if giving repeated annual doses of the experimental drug CD388 is safe and how the body reacts to it in healthy adults who have already received one dose without serious side effects. The study aims to determine if the body makes antibodies against CD388 after repeated doses, which might affect how the drug works or how safe it is, and to better understand the safety and tolerability of repeated doses. Participants will receive two doses of CD388 over two years and be monitored for 18 months. Researchers will check for immune responses against the drug, watch for any side effects, and measure how the drug behaves in the body over time. This study is based on the idea that people who tolerated CD388 well before will likely continue to tolerate it safely with repeated annual dosing, and that the risk of immune reactions will remain low. Expanded access to the study drug will not be provided to participants after the study ends.