[jyl]

It isn't an immunity thing.

Paxlovid (nirmatrelvir) is taken with ritonavir to slow the body's clearance of nirmatrelvir, which is the active anti-viral drug, otherwise body would clear the nirmatrelvir too quickly (half life of nirmatrelvir in body, without ritonavir, is pretty short). Ritonavir is used in other combo drugs for the same purpose.

However, ritonavir also slows body's clearance of other medications that people sometimes take, so potentially over the week course of Paxlovid those other medications could potentially build up to too-high levels in the body.

Doctors could instruct patients to interrupt or reduce dosing of those other medications while taking Paxlovid, e.g. take XYZ every n days instead of daily. Since that is complicated, they often prescribe Lagevrio (molnupiravir) instead. The problem is that Lagevrio is not as effective than Paxlovid, by quite a margin. So patients are getting a less-effective antiviral.

Still, much better to take Lagevrio than nothing!