As a caver i've come across carbon dioxide a few times. Levels have been as high a 5.1%! At 3% one breathes very heavily which is an excellent warning. From memory Australian workplace regulations (coal mines) dictated 0.5% as a maximum. CO2 detectors should not be too hard to organise. Aircrew must all have suitable O2 readily available for the duration of flight.
Initially vaccines will be on vaccine only freighters. Later on they may be with mixed cargo but no animals. Further down the track even on regular pax flight. In the event of a CO2 build-up a depressurise and divert emergency procedure would apply.