Another variable that causes issues in the results is the effectiveness of the vaccine and the evolution of the virus over years with retained immunity.

For example:

A flu shot from years ago can protect against a flu from this year or in the future. So even if the flu vaccine isn’t effective in its current year, the immunity lasts for many years to those antigens. The strain of flu that’s currently dominant can fluctuate to newer or older variants. Controlling for this is even more confusing

A guess this year could help in future years