SCHEDULE 13D/A 0000950123-11-055171 0001472789 XXXXXXXX LIVE 19 Common Shares, par value US$0.001 per share 07/10/2025 false 0001084201 P8696W104 SINOVAC BIOTECH LTD.

No. 39 Shangdi Xi Rd Haidian District Beijing F4 100085

Andrew Y Yan 852 2918-2200 c/o Suite 1102, Five Pacific Place 28 Hennessy Road, Wanchai Hong Kong K3 00000 0001472789 N SAIF Partners IV L.P. b WC N E9 10780820.00 0.00 10780820.00 0.00 10780820.00 N 15.00 PN Comments to item 13: Calculation is based on 71,860,702 Common Shares of the Issuer outstanding as of March 31, 2024 according to the Issuer's annual report on Form 20-F for the fiscal year ended December 31, 2023 (the "2023 Annual Report"). Y SAIF IV GP, L.P. b WC N E9 10780820.00 0.00 10780820.00 0.00 10780820.00 N 15.00 PN Comments to item 13: Calculation is based on 71,860,702 Common Shares of the Issuer outstanding as of March 31, 2024 according to the 2023 Annual Report. Y SAIF IV GP Capital Ltd. b WC N E9 10780820.00 0.00 10780820.00 0.00 10780820.00 N 15.00 CO Comments to item 13: Calculation is based on 71,860,702 Common Shares of the Issuer outstanding as of March 31, 2024 according to the 2023 Annual Report. Common Shares, par value US$0.001 per share SINOVAC BIOTECH LTD. No. 39 Shangdi Xi Rd Haidian District Beijing F4 100085 This Amendment No.19 to Schedule 13D is being filed on behalf of each of SAIF Partners IV L.P., SAIF IV GP, L.P., and SAIF IV GP Capital Ltd. (each, a "Reporting Person," and collectively, the "Reporting Persons"), to amend the Schedule 13D relating to the common shares, par value $0.001 per share (the "Common Shares"), of Sinovac Biotech Ltd. (the "Issuer"), filed by the Reporting Persons on May 31, 2011, as amended by Amendment No.1 to Schedule 13D filed on August 23, 2011, Amendment No.2 to Schedule 13D filed on September 30, 2011, Amendment No.3 to Schedule 13D filed on November 29, 2011, Amendment No.4 to Schedule 13D filed on December 30, 2011, Amendment No.5 to Schedule 13D filed on February 2, 2016, Amendment No. 6 to Schedule 13D filed on June 26, 2017, Amendment No. 7 to Schedule 13D filed on December 12, 2022, Amendment No. 8 to Schedule 13D filed on March 15, 2023, Amendment No. 9 to Schedule 13D filed on May 25, 2023, Amendment No. 10 to Schedule 13D filed on September 15, 2023, Amendment No. 11 to Schedule 13D filed on December 1, 2023, Amendment No. 12 to Schedule 13D filed on June 17, 2024, Amendment No. 13 to Schedule 13D filed on September 12, 2024, Amendment No. 14 to Schedule 13D filed on February 19, 2025, Amendment No. 15 to Schedule 13D filed on March 19, 2025, Amendment No. 16 to Schedule 13D filed on April 28, 2025, Amendment No. 17 to Schedule 13D filed on May 23, 2025, and Amendment No. 18 to Schedule 13D filed on June 17, 2025 (collectively, the "Original Schedule 13D"). The information in each Item below amends the information disclosed under the corresponding Item of the Original Schedule 13D. Except as amended and supplemented in previous amendments and herein, the information set forth in the Original Schedule 13D remains unchanged. Capitalized terms used herein have meanings as assigned thereto in the Original Schedule 13D unless defined herein. The information set forth in response to each separate Item below shall be deemed to be a response to all Items where such information is relevant. Item 4 is hereby amended and supplemented to add the following: The Issuer disclosed on July 10, 2025 a new board of directors of the Issuer (the "Board") comprising Mr. Simon Anderson, Mr. Shan Fu, Mr. Shuge Jiao, Mr. Chiang Li, Mr. Yuk Lam Lo, Mr. Yumin Qiu, Mr. Yu Wang, Ms. Rui-Ping Xiao, Mr. Andrew Y Yan (as Chairman of the Board) and Mr. Yin Weidong. The Reporting Persons have also engaged and will continue to engage in discussions with other shareholders of the Issuer regarding the Board, the corporate governance of the Issuer and similar matters that the Reporting Persons believe would increase shareholder value, as well as pursue other plans or proposals that relate to or would result in any of the matters set forth in clauses (a)-(j) of Item 4 of Schedule 13D. The filing of this Amendment shall not be construed as an admission that the Reporting Persons and their affiliates, on the one hand, and any other shareholders of the Issuer and their affiliates, on the other hand, are a group, or have agreed to act as a group with each other for purposes of Section 13(d) of the Act or for any other purpose. The Reporting Persons expressly disclaim beneficial ownership of the common shares beneficially owned by other shareholders who may also be advocating for changes to the Board. Item 6 is hereby amended and supplemented to add the following: Item 4 of Amendment No. 19 is incorporated herein by reference. SAIF Partners IV L.P. /s/Andrew Y Yan 07/14/2025 SAIF IV GP, L.P. /s/Andrew Y Yan 07/14/2025 SAIF IV GP Capital Ltd. /s/Andrew Y Yan 07/14/2025