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Ivermectin does not directly prevent antibodies from being formed. At least I have not heard of any evidence in that direction.
Ivermectin is a general anti-viral and anti-inflammatory medicine. When used as a COVID preventive, if an initial infection event does not result in a sustained infection, then there could be little anti-body response. But when Ivermectin is used to treat an active COVID infection, then the established viral level should allow robust antibody generation.