How good is Napkin Calculation #3 of the Delta variant breakthrough rate? As napkin calculations are partly intuitive, let’s compare. The media have provided a variety of numbers which are not comparable. Breakthrough that counts only symptomatic cases is not the same as breakthrough of test-positive cases.

A Scotland study has a comparable result: (The official journal of The Royal Pharmaceutical Society) Second dose of Pfizer and Oxford vaccines offer reduced protection against Delta variant of COVID-19, study suggests. Quoting,

Results of a Scotland-wide study have shown that the Pfizer/BioNTech COVID-19 vaccine provides 79% protection against the Delta variant two weeks after the second jab, while the second dose of the Oxford/AstraZeneca vaccine offers 60% protection.

From Delta Strain of COVID — We’re in for a Rough Ride; Napkin Calculation #3,

A breakthrough infection percentage of between 5X and 10X the percentage of hospitalized patients, breakthrough of 20%-40%. compared to 5% for the viruses of the original Phase 3 study.

79% for Pfizer, implying 21% breakthrough, is within the range of 20-40% predicted by the napkin calculation.  The Pfizer results indicate great benefit for the immunized individual. Whether the Pfizer shot will adequately suppress community transmission remains to be seen.