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You are fortunate to still have test kits available to you. If you are not immunocompromised, then after 15 days you are unlikely to still be infectious. The reason I bring up immune status (immunocompetent vs. immunocompromised) is because we have seen that those with immune status that has been compromised (e.g., cancer or transplant patients) can continue to shed the virus for months and have a prolonged illness period.