"The number of new infections is being kept low." "the number of ~" 「〜の数」 "new infections" 「感染者」"infected people" も言えます。 "is being kept ~" 「〜て抑えられています」 "low" 「低く」 "is being kept low" 「低く抑えられています」"is being contained" や "is being kept under control" も使えます。