This whole thing is stupid. Of course immunity doesn't last forever, but a side effect of continually getting exposed to a virus is that your body is continually updating its immunity to it. Under normal circumstances, you're going to be exposed to the exact same virus over and over due to natural contact with others. Thats why people in rest homes that live in a much more secure environment are way more likely to die from the flu. The more you isolate yourself, the fewer opportunities your body has to regenerate antibodies while still having any level of them. Its not like under normal circumstances you catch a virus, build immunity, and that immunity times out after x number of days.