Study limitations
This is a retrospective study, which means that it depends on data that are entered into a clinical database, not collected for research and conducted in a single hospital centre, the study is settled in a single hospital centre, which may lead to difficulties when trying to observe clinical differences between groups if there is a loss of data, yet this problem was addressed by previous sample size calculations to ensure data obtained were representative and can be consulted in supplementary material. Also, certain variables that have the potential to impact the outcome may not have been recorded at all 38.
Sample size varies between 3 and 6 doses timepoints. This loss of follow-up is due to the pandemic situation precipitated by COVID-19. Since this is a retrospective study, these data could not be recovered when processed. Nevertheless, the statistical outcome is solid and significative, yet the greater reduction observed after 6 doses should be addressed with caution and considering that no statistically significant difference between 3 and 6 doses was observed despite the reduction observed after 6 doses was greater than the one achieved after 3.