In the early days of the COVID-19 vaccine rollout, Chinese shots saved countless lives. They kick-started inoculation programs across Asia, Latin America and the Middle East, while richer countries hoarded scarce mRNA shots from Pfizer Inc. and Moderna Inc.

But many governments that once relied on vaccines from Sinovac Biotech Ltd. or Sinopharm Group Co Ltd. are now turning to options from the U.S. and Europe instead, as concerns mount about Chinese vaccines’ efficacy against the delta strain and as the Western stranglehold on mRNA supplies loosens.

That preference may already be showing up in China’s customs data, where exports of human vaccines dropped 21% in August to $1.96 billion from $2.48 billion in July after having risen steadily since Dec. 2020.