That study only measured immunity for 90 days. I would have expected all covid cases to offer at least 90 days of immunity, but in this study it did not. At 90 days .03% of the previously infected tested positive again.
Only 3% of people who had not previously had covid tested positive.
And the numbers from the nursing home study which was an entirely different study were particularly disappointing. Out of 12 people who had covid in July, 5 of them had it again in October and in each case they had a worse case.
We’ll have to agree to disagree on what “powerful immunity” is...:-)
I’d caution against considering that report of a handful of reinfections from a nursing home as a study - it is an anecdote. There were no controls, no analysis to weed out confounding factors (most nursing home residents, almost by definition, are at death’s door, with multiple co-morbidities), and far from a statistically significant sample size.
In general, the older people get, the less effective their immune systems are. That is a hallmark of aging, across all infectious diseases. No surprise.
You mention that at 90 days, .03% of previously infected tested positive again, but “only” 3% of those who had not previously had COVID did.
99% reduction is generally seen as quite strong protection.