The incidence rate ratio (IRR) comparing natural immunity to vaccine immunity was 0.60 (95% confidence interval [CI] 0.12,1.83; P = 0.55) and the IRR comparing the combination of vaccine immunity and natural immunity to vaccine immunity was 0.25 (95% CI 0.01,1.43; P = 0.19). In sensitivity analyses restricting to patients aged above 50 years and above 65 years, results were consistent with the main analysis indicating the highest incidence rate in the vaccine immunity group and lowest incidence for the combination of vaccine immunity and natural immunity.
Most of the patients with natural and vaccine immunity (57%) and vaccine immunity only (63%) received the Pfizer-BioNTech vaccine, followed by the Moderna vaccine (36% and 32%, respectively). During a follow-up time of 21 calendar months, out of 106,349 primary care patients (mean age, 52.3 years; 56.6% were female), there were 69 breakthrough COVID-19 hospitalisations. Of these, 65 (0.06%) occurred among 102,613 patients with vaccine immunity, 3 (0.03%) occurred among 11,047 patients with natural immunity, while the remaining 1 (0.01%) patient was among 7,313 patients with both natural and vaccine immunity.