One in five pupils from poorer backgrounds were persistently absent from school last year.
Figures show 10% of all pupils were persistently absent during the pandemic - the rate was under 3% before that.
The absence gap between pupils eligible for free school meals (FSM) and those who are not grew after Covid, the Welsh government figures suggest.
One teaching union said some pupils were vulnerable to missing school, having disengaged during lockdown.
Persistent absence is when a pupil misses than 20% of classes over the school year.
For the first time, these figures compare the last academic year - when pupils went back to school - with the five years before the pandemic.
