Serum Institute of India has sought permission from India's drug regulator to conduct a phase-3 study of its COVID-19 vaccine Covovax as a booster dose in adults, official sources said on Sunday. The Drugs Controller General Of India (DCGI) had approved Covovax for restricted use in emergency situations in adults on December 28. The vaccine is yet to be included in the country's inoculation programme. Prakash Kumar Singh, Director, Government and Regulatory Affairs at SII had last week submitted an application to DCGI seeking nod for phase-3, observer-blind, randomised, controlled study to evaluate the safety and immunogenicity of Covovax as booster dose on those who have received primary vaccination either with Covishield or Covaxin at least three months ago, an official source said.