New Delhi: Serum Institute of India (SII), the world's largest vaccine manufacturer by volume, has sought the central government's intervention so as to enable the firm import essential raw materials from the US for uninterrupted manufacturing and supply of COVID-19 vaccines.In a letter to Commerce Secretary Anup Wadhawan and Foreign Secretary Harsh Vardhan Shringla, Director, Government and Regulatory Affairs at Pune-based SII, Prakash Kumar Singh said the US government has invoked the Defence Production Act because of which the firm is facing difficulties in importing necessary products like cell culture medias, raw material, single-use tubing assemblies and some specialty chemicals from the US.He said in the letter that Covishield.