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.