X0306 4 2020-06-30 0 0001805890 Fusion Pharmaceuticals Inc. FUSN 0001609405 Khuong Chau Quang C/O FUSION PHARMACEUTICALS INC. 270 LONGWOOD ROAD SOUTH HAMILTON A6 L8P 0A6 ONTARIO, CANADA 1 0 0 0 Common Shares 2020-06-30 4 C 0 2224770 A 2224770 I See Footnote Common Shares 2020-06-30 4 P 0 705900 17.00 A 2930670 I See Footnote Common Shares 2020-06-30 4 P 0 192100 17.00 A 192100 I See Footnote Class B Preferred Shares 2020-06-30 4 C 0 11878051 D Common Shares 2224770 0 I See Footnote The Class B Preferred Shares were convertible into Common Shares on a one-for-5.339 basis. Upon the closing of the Issuer's initial public offering, all Class B Preferred Shares converted into Common Shares. OrbiMed Capital GP VII LLC ("GP VII") is the general partner of OrbiMed Private Investments VII, LP ("OPI VII"). OrbiMed Advisors LLC ("OrbiMed Advisors") is the managing member of GP VII. By virtue of such relationships, GP VII and OrbiMed Advisors may be deemed to have voting and investment power with respect to the shares held by OPI VII and as a result may be deemed to have beneficial ownership of such shares. The Reporting Person is an employee of OrbiMed Advisors. OrbiMed Capital LLC ("OrbiMed Capital") is a relying advisor of OrbiMed Advisors. OrbiMed Capital and OrbiMed Advisors exercise investment and voting power through a management committee comprised of Carl L. Gordon, Sven H. Borho and Jonathan T. Silverstein. Each of OrbiMed Capital, GP VII, OrbiMed Advisors, and the Reporting Person disclaims beneficial ownership of the shares held by OPI VII, except to the extent of its or his pecuniary interest therein, if any. OrbiMed Capital is the investment advisor to OrbiMed Partners Master Fund Limited ("OPM"). OrbiMed Capital is a relying advisor of OrbiMed Advisors. OrbiMed Advisors and OrbiMed Capital exercise investment and voting power through a management committee comprised of Carl L. Gordon, Jonathan T. Silverstein, and Sven H. Borho, each of whom disclaims beneficial ownership of the shares of OPM. /s/ John Crowley, Attorney-in-Fact 2020-07-02