Not a small number of patients. Just turns out the number of patients in the vaxxed group or previous infected group both did well and few cases were reported in either group but natural immunity group performed better. To say this wasn't a large enough study is not the case.
Model 1 compared 16,215 people in both the vaccinated and natural immunity groups and found that cases in the vaccinated group (n=238, 1.5%) were 13 fold more likely to experience a breakthrough infection than the natural immunity group (n=19, 0.12%). The majority of the cases were symptomatic. There were very few hospitalizations in either group with only 8 in the vaccine arm and 1 in the natural immunity arm.
Model 2 compared 46,035 people in both the vaccinated and natural immunity groups, and found that cases in the vaccinated group (n=640, 1.4%) were 6-fold more likely to experience a breakthrough infection than the natural immunity group (n=108, 0.23%). The majority of the cases were symptomatic. There were very few hospitalizations in either group with only 21 in the vaccine arm and 4 in the natural immunity arm.