Overall mortality tends to decline in recessions, so that could contribute too.

Yup. This also was also reported to have provided a weaker than usual signal for the seasonal Flu vaccine in the North.

Each hemisphere uses data collected during the winter in the other hemisphere to guess what Influenza variants will be most common in their countries when it's their turn, then it takes several months to turn those guesses into bulk vaccine for immunising the elderly and vulnerable and give them a jab. Sometimes the guess is right on the money and it's extremely effective, other years not so much.

Less signal may make this year's winter 2020/21 vaccine less effective.

> Sometimes the guess is right on the money and it's extremely effective, other years not so much.

Oh thats why some years it seems to work, others not, TIL.