"Take a Closer Look", page-512

Well yeah. Naive projections are rarely meaningful, but it does mean SOMETHING.

Things it does mean:
1. That subsequent boosters are very likely to further increase immunity
2. That subsequent increases in immunity could potentially take us over 75% reduction

Things it does not mean:
1. That subsequent boosters will continue to increase immunity at the same rate the first two have, or even at all.

2. That 100% reduction is the maximum we are approaching. Even if boosters continue to cumulatively reduce shedding by the same percentage per-booster indefinitely, It is possible we are approaching a different maximum which cannot be surpassed unless the disease is cured completely.