Capacity restrictions were enforced by the US on Chinese carriers.

As five passengers who traveled from San Francisco to Shanghai on July 21 tested positive for covid-19. The Chinese government imposed a curb on four united airlines flights followed by which US department of transportation declared on 18th August that they will limit some flights of Chinese carriers to 40% passenger capacity for four weeks starting from 23 August.

                       Courtesy: http://planespotters.net

Earlier Chinese authorities also gave United three options i.e. either they cancel two San Francisco to Shanghai flights or operate the two flights without passengers or else operate four flights with up to 40% of passenger capacity.

There is a strong long-standing air agreement between China and the United States that allows both countries to operate more than 100 weekly flights between two nations but only a fraction of those are currently operating.