Read this:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6883732/
"The traditional teaching was that if a patient’s biologic therapy was interrupted and his or her drug level dropped too low, restarting the therapy carried a high risk of developing antidrug antibodies and, therefore, losing response to the therapy. Thus, the dogma was that once the patient started the therapy, he or she needed to stay on it and be adherent to the maintenance regimen. The current thinking is that when a patient has a drug holiday, whether it is intentional or unintentional, and the drug level drops to zero in between dosing, the need to restart therapy is based on 2 factors: clinical recurrence of the disease (ie, the patient has a relapse and needs to be treated actively) or, in the modern era, subclinical recurrence (ie, a doctor is monitoring the patient and detects inflammation before symptoms develop). Those are the times when treatment needs to be restarted, whether that means going back to a therapy the patient has already been on (and from which the patient is currently on a drug holiday) or moving to a new therapy (possibly with a novel mechanism of action). This decision should be discussed very carefully."
The bottom line is a high risk of antibodies developing, and antibodies is not a "side effect", it is "treatment is done and over", as in "kaput."