SCHEDULE 13D/A 0000950123-11-055171 0001472789 XXXXXXXX LIVE 18 Common Shares, par value US$0.001 per share 06/16/2025 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.18 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, and Amendment No. 17 to Schedule 13D filed on May 23, 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: On June 16, 2025, SAIF Partners IV L.P. ("SAIF") mailed its definitive proxy materials to shareholders of the Issuer, in connection with the special meeting of shareholders of the Issuer to be held on Tuesday, July 8, 2025 at 8 p.m. Atlantic Standard Time (Wednesday, July 9, 2025 at 8:00 a.m. China Standard Time), and at any adjourned, postponed, continued, or rescheduled meeting thereof (the "Special Meeting"), to solicit proxies to elect Simon Anderson, Shan Fu, Shuge Jiao, Chiang Li, Yuk Lam Lo, Yumin Qiu, Yu Wang, Rui-Ping Xiao, Andrew Y Yan, and Weidong Yin to the board of the directors (the "Board") of the Issuer, and to remove David Guowei Wang, Sven H. Borho, and any other person or persons elected or appointed to the Board without shareholders' approval after February 8, 2025 and up to and including the date of the Special Meeting from the Board and each applicable committee thereof. The proxy statement mailed by SAIF is filed as Exhibit 1 hereto and is incorporated herein by reference. The Reporting Persons have also engaged in preliminary discussions with other shareholders of the Issuer regarding its proposal to remove the above-referenced existing members of the directors from the Board and to elect the above-referenced nominees to the Board. The Reporting Persons plan to continue communicating with the Issuer and with other shareholders regarding these and similar matters. Depending upon, among other things, the outcome of these communications, the financial condition, results of operations and prospects of the Issuer, conditions in the securities markets, general economic conditions and other factors that the Reporting Persons deem relevant, the Reporting Persons may take other steps seeking to bring about changes to the Board and/or the Issuer that it believes 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 Item 4 of Schedule 13D. Item 6 is hereby amended and supplemented to add the following: Item 4 of Amendment No. 18 is incorporated herein by reference. Exhibit 1 Definitive Proxy Statement SAIF Partners IV L.P. /s/Andrew Y Yan 06/16/2025 SAIF IV GP, L.P. /s/Andrew Y Yan 06/16/2025 SAIF IV GP Capital Ltd. /s/Andrew Y Yan 06/16/2025