stylin19a points out that I mis-read the chart.
Both of you are correct. I apologize for mis-reading it, despite the clear explanation at top. Disregard my earlier post.
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I was trying to determine how many persons who were age 21 in 1899 (born in 1878) might have survived another 50 years, to attain age 71 in 1949.
Actuarial chart: the relevant data is in row three. We can ignore the first two rows, since they were based on Massachusetts only.
By the turn of the century, 20-year-olds could expect to live about another 40 years. Persons born ~ 1878 likely had a shorter range. Some might have survived to 1949, but there's no way to derive how many.
Can't extrapolate from that chart the fraction of a group with a lifeline of 60 that might live another 18.3% to 71.
From that small pool of 71-year-old survivors, the probability that at least one vaxxed geezer who landed in the Rio Grande Valley – and somehow close to the eight smallpox victims – is the example verifying that the smallpox inoculation gave him "durable immunity" becomes vanishingly small, but I can't quantify it.