China has been imposing air, and even maritime, travel restrictions against other countries, especially the weaker ones that include the Philippines, in the South China Sea (SCS) for years, but has not officially declared its enforcement of its air defense identification zone (ADIZ), perhaps for the sake of diplomacy and security. But Beijing, now under intense international scrutiny over the Covid-19 pandemic and its origins, faces the prospects of global isolation and further backlash if it so decides to admit to the enforcement of the ADIZ, although it has been in effect and was already being practiced against states—with no less than Manila experiencing it for the last seven years. Still, if and when it is officially declared, the ADIZ would allow China to clamp security around the military bases it has built on reclaimed islands in the regional waters by restricting even commercial overflights, while already constricting maritime access to the waters through which nearly half of the world’s trade passes annually.

Acosta, R. (2020, July 4). 'In effect, but undeclared'. Business Mirror, pp. A1, A2.