Although no significant differences in the serum ATX levels and ⊿ATX were observed among the three groups classified according to the COVID-19 severity, the negative correlations of the serum ATX levels with the serum CRP and D-dimer levels on several of the specified days after the onset of infection (Figure 3 and Tables 2 and 3) suggested the possibility that the decline of the serum ATX levels might represent a compensatory response to protect against COVID-19. Here, ENPP2 is linked to infection.