(1) C5a levels of pneumonia (non-COVID-19) patients decreased significantly (p = 0.004), with a large effect size (d = 0.77), but C5a levels of ARDS (COVID-19) patients increased without significance (p = 0.068) and with a moderate effect size (d = 0.38) for at least 10 days after the beginning of hospital care. This evidence concerns the gene C5 and COVID-19.