You have to consider what percentage were vaccinated anyway before you can try to measure the impact. I think it is less a specific number of days and more comparing cancelled and delayed flights and baggage with and without the policy.
From a thread on an aviation forum two weeks ago, there was a day where United cancelled 9% of the scheduled flights, Delta 6%, JetBlue 5% and American fewer than 1%.
The difference at American wasn't vaccination rate. It was that they offered 300% pay to flight attendants who were off to work and cover the call outs.
Doesn't that very accurately show the economic impact?
The numbers aren't perfectly comparable, since we don't know how lean vs how much cushion each airline operates with. Based on the different airline stereotypes, we can probably assume that United runs with less backup. Not because they're just better, but because they're cheap.
So, now you have to ask, for each of those, compared to itself, what would the impact be if they had the opposite vaccination policy? Would United have had more cancellations, would Delta and Jet blue had different, and would American had to spend more or less money?
I think it's a fairly safe guess that United would have had more cancelations if everything was the same and only the vaccination policy was different. If only because more people would be out at once, since those out would be out for longer durations.
For American Airlines, if everyone was vaccinated, and hence less people out for longer, would they have needed to offer 300% pay to as many people? That's not even considering the operations impact of juggling all the scheduling changes and backfill process, just the direct pay number.
From a business impact question, it's not difficult. Was the employee loss from the vaccine mandate and any costs of getting new employees more or less than the cost of cancelling flights, paying some 300%, and the increased operations cost created? As a bonus, the first is a one time impact while the second is ongoing and will ebb and flow as waves ebb and flow.
I don't disagree that the employee vaccine mandate had a cost. Both administrative (which was probably cheap) and with employee turnover (which was less than 1% if I remember). Comparing those costs, if the impact was one weekend or one week, maybe it would be wash. But, it's been going on a month now, destabilizing all of the airline operations. That's a huge cost.
I'm sure the United CEO did the math (his actuarial and accounting teams anyway) and determined the one time cost was less than the ongoing operations cost were likely to be. He's not going to do news releases and employee messages about how he did the math and it's cheaper to require them to be vaccinated than not and it's just a nice side effect they're les likely to die, especially when viewed as interchangeable resources instead of as people. Hence a nice sunny message about how it's all good for them.
The public health implications don't have to matter or be considered at all in the calculation.