METHODS
Study design and setting
We performed a cohort study to compare the effectiveness of homologous versus heterologous vaccination against SARS-CoV-2 infection. Vaccination status information came from the Catalan Shared Clinical Records, a database with data from primary care practices and hospitals of the whole Catalan health system. We further linked data to the Catalan database of RT-PCR and lateral flow test (LFT) for SARS-CoV-2 and primary care electronic health records (EHR). Data from these databases have been previously used for many studies of covid-19 and include information of nearly 90% of the Catalan population [Cabezas C et al. BMJ 2021].
Participants, cohorts and follow-up
The main cohort considered included all individuals that received a first dose of ChAdOx1 vaccine and then received a second dose of ChAdOx1 (homologous vaccination) or BNT162b2 (heterologous vaccination). In Catalonia, ChAdOx1 vaccine was initially targeted to essential non-healthcare workers and other social priority groups between 18 and 55 years old. After a temporary halt in March 2021, ChAdOx1 vaccine was extended to those aged between 60 and 69. At the time of the second dose, individuals could choose if they received a second dose of ChAdOx1 or a heterologous ChAdOx1/BNT162b2 combination. Given that nearly all individuals aged 60-69 choose a homologous vaccination schedule we included only individuals aged between 18 and 60 due to potential selection biases. We also excluded those with a previous SARS-CoV-2 infection identified by a positive RT-PCR or LTF and those who were not assigned to one of the primary care practices contributing to our database.
Each individual with an heterologous combination was matched with and individual with homologous vaccination schedule on age, sex, healthcare area of residence, and date of second dose. We followed both cohorts from the day they received the second dose until an outcome, death or the end of the study (13 October 2021).