Exhibit 99.1

Eiger BioPharmaceuticals Reports First Quarter 2021 Financial Results and Provides Business Update

Palo Alto, Calif., May 6, 2021 /PRNewswire/ — Eiger BioPharmaceuticals, Inc. (Nasdaq:EIGR), a commercial-stage biopharmaceutical company focused on the development and commercialization of targeted therapies for serious rare and ultra-rare diseases, today reported its first quarter 2021 financial results and provided a business update.

“The Phase 3 D-LIVR study is now 75% enrolled, and by year-end we expect to complete enrollment of this landmark global study advancing Lonafarnib, the only oral agent in development for HDV, setting the stage for Week 48 end of treatment data in 2022,” said David Cory, President and CEO of Eiger. “In parallel, we will initiate the Phase 3 LIMT-2 study in 2021 to advance Lambda, a well-tolerated interferon, also toward registration for HDV. Most recently, with a goal of developing a convenient, outpatient treatment for COVID-19, we announced that a Lambda arm will be included in the ongoing, multi-center, Phase 3 TOGETHER platform study in Brazil.”

Program Updates and Upcoming Milestones

Lonafarnib for HDV

Peginterferon Lambda (Lambda) for HDV

Zokinvy^® (lonafarnib) for Progeria

Lambda for COVID-19

Avexitide for Post-Bariatric Hypoglycemia (PBH) and Congenital Hyperinsulinism (CHI)

Corporate

First Quarter 2021 Financial Results

Eiger reported total net revenue of $3.6 million from Zokinvy product sales for first quarter 2021. Zokinvy launched commercially in the U.S. in January 2021 with first quarter sales including inventory stocking at the specialty pharmacy.

Research and Development expenses were $13.8 million for first quarter 2021 as compared to $9.5 million for the same period in 2020. The increase was primarily due to clinical trial related expenses, including contract manufacturing, and headcount related expenses, including stock-based compensation expense. The increase was partially offset by lower expenses for regulatory consulting services.

Selling, General and Administrative expenses were $5.6 million for first quarter 2021, as compared to $5.2 million for the same period in 2020. The increase in first quarter 2021 was primarily due to outside consulting and advisory services.

Total operating expenses include non-cash expenses of $2.2 million for first quarter 2021, as compared to $2.0 million same period in 2020.

Eiger reported first quarter 2021 net income of $29.2 million, or $0.85 on a fully-diluted per share basis. This compares to a net loss of $15.2 million, or $0.62 on a per share basis, for first quarter 2020. First quarter 2021 net income was driven by a one-time gain from the sale of the Zokinvy Priority Review Voucher (PRV). The company reported $45.9 million as Other Income, Net, which primarily reflects net PRV sale proceeds.

Cash, cash equivalents, and investments as of March 31, 2021 totaled $160.5 million compared to $128.8 million on December 31, 2020.

As of March 31, 2021, the company had 34.0 million of common shares outstanding.

Conference Call

At 4:30 p.m. Eastern Time today, Eiger will host a conference call to discuss its financial results and provide a business update. The live and replayed webcast of the call will be available through the company’s website at www.eigerbio.com. To participate in the live call by phone, dial (844) 743-2495 (U.S.) or (661) 378-9529 (International) and enter conference ID 1090136. The webcast will be archived and available for replay for at least 90 days after the event.

About Eiger

Eiger is a commercial-stage biopharmaceutical company focused on the development and commercialization of targeted therapies for serious rare and ultra-rare diseases.

Eiger’s lead clinical programs are focused on the development of foundational therapies for Hepatitis Delta Virus (HDV) infection, the most serious form of viral hepatitis, with two complementary HDV treatments. Lonafarnib is a first-in-class, oral prenylation inhibitor in a global Phase 3 trial. Peginterferon lambda is a first-in-class, well-tolerated type III interferon entering Phase 3.

Zokinvy^® for the treatment of Hutchinson-Gilford Progeria Syndrome (HGPS or Progeria) and processing-deficient progeroid laminopathies is the Company’s first FDA approved product. A Marketing Authorization Application (MAA) is under review by the European Medicines Agency (EMA).

For additional information about Eiger and its clinical programs, please visit www.eigerbio.com.

Note Regarding Forward-Looking Statements

This press release contains “forward-looking” statements that involve substantial risks and uncertainties. All statements other than statements of historical facts, including statements regarding our future financial condition, timing for and outcomes of clinical results, business strategy and plans and objectives for future operations, are forward-looking statements. These forward-looking statements include terminology such as

“believe,” “will,” “may,” “estimate,” “continue,” “anticipate,” “contemplate,” “intend,” “target,” “project,” “should,” “plan,” “expect,” “predict,” “could,” “potentially” or the negative of these terms. Forward-looking statements are our current statements regarding our intentions, beliefs, projections, outlook, analyses or current expectations concerning, among other things, our anticipated significant milestones in 2021; the timing of our ongoing and planned clinical development; the sufficiency of our cash, cash equivalents and investments to fund our operations into the fourth quarter of 2023; our development programs for Zokinvy generally; and the potential approval of Zokinvy in jurisdictions outside of the U.S., including the EU; our progression and continued enrollment of our Phase 3 D-LIVR study in HDV; our ability to maintain supply of our commercial and clinical trial materials; our plans to advance Lambda in HDV in the U.S. and EU; our progression of Lambda for COVID-19 and Avexitide for PBH and CHI; our ability to finance the continued advancement of our development pipeline products; and the potential for success of any of our product candidates. These statements concern product candidates that have not yet been approved for marketing by the U.S. Food and Drug Administration (FDA). No representation is made as to their safety or effectiveness for the purposes for which they are being investigated. Various important factors could cause actual results or events to differ materially from the forward-looking statements that Eiger makes, including additional applicable risks and uncertainties described in the “Risk Factors” sections in the Quarterly Report on Form 10-Q for the quarter ended March 31, 2021 and Eiger’s subsequent filings with the SEC. The forward-looking statements contained in this press release are based on information currently available to Eiger and speak only as of the date on which they are made. Eiger does not undertake and specifically disclaims any obligation to update any forward-looking statements, whether as a result of any new information, future events, changed circumstances or otherwise.

SOURCE Eiger BioPharmaceuticals, Inc.

Investors and Media:

Sri Ryali

Email: sryali@eigerbio.com

Phone: 1-650-272-6138

Eiger BioPharmaceuticals Inc.

Condensed Consolidated Balance Sheets

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Eiger BioPharmaceuticals Inc.

Condensed Consolidated Statements of Operations Financial Data

(in thousands, except per share and share amounts)