Pretty simple - the more vaccinated the more immune systems compromised - that's why the flu and other strains are more deadly. Might in for a [bleep] we haven't experienced in a while. Only thing I've learned is that the institutions supposed to handle things like this all failed miserably.

"The first wave, called the Black Death in Europe, was from 1347 to 1351. The second wave in the 1500s saw the emergence of a new virulent strain of the disease".