Is there a maximum magnetic field in QED?

It was recently conjectured by Shabad and Usov that there exists in QED a maximum magnetic field of 10^{42} G, above which the magnetized vacuum becomes unstable. Using a nonperturbative analysis that consistently incorporates the effective electron mass and the screening effect in a strong magnetic field, we show that the conjectured phenomenon of positronium collapse never takes place. Thus, there does not exist a maximum magnetic field in QED and the magnetized vacuum is stable for all values of the magnetic field.