"The three major pathotype shifts are:
(1) from m MDV to v MDV strains in the late 1950s;
(2) from v MDV to vv MDV in the late 1970s;
(3) the appearance of the putative vv + pathotype in the early 1990s (Figure 4).
Each occurrence has been associated with greater disease losses which have persisted until introduction of the next generation of vaccine. The reason for the shifts remain speculative. Shifts in the 1970s and 1990s may have been in response to certain MD vaccines. Kreager (1996) has suggested vaccines may have only a 10-year useful life before new strains evolve. The shift in the 1950s occurred prior to vaccine use, but may have been related to changes in poultry husbandry, especially the move towards larger houses and the increasing density of poultry operations in certain geographic regions"
m = mild, v = virulent, vv = very virulent, vv+ = very virulent plus, as explained on page S46, Table 1.
To be very clear, Marek is not covid. I am drawing no equivalence. I just point out that Andrew Read's 2021 articlet is built around a weak argument.