April 30 (Reuters) - American depositary shares of Vaccitech, which co-invented the COVID-19 vaccine developed by AstraZeneca Plc and Oxford University, fell nearly 20% in their market debut on Friday, giving the biotech firm a market value of about $464 million. The company’s shares opened at $13.62, below the initial public offering price of $17. (Reporting by Vishwadha Chander in Bengaluru; Editing by Aditya Soni)