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The biopharma sector weathered the COVID-19 pandemic fairly well, with the Shares Nasdaq Biotechnology ETF IBB outperforming the S&P 500 Index. This defensive sector also benefited from opportunity in the form of SARS-CoV-2, the virus that causes COVID-19.

It, therefore, comes as a no surprise that two of the top five gainers among biotech stocks are vaccine developers. Here are the biotechs that advanced in excess of 1,000% thus far this year:

Novavax, Inc. (NVAX) (Gain: +2799%)

Much of Novavax's rally is attributable to its coronavirus vaccine program. The company identified its investigational vaccine, codenamed NVX-CoV2373, in early April. It is a stable, prefusion protein made using the company's proprietary nanoparticle technology, with Matrix-M adjuvant added to it.

Ahead of the initiation of the coronavirus vaccine program, the shares received a lift from positive developments concerning its influenza vaccine candidate NanoFlu. After proceeding seamlessly with its vaccine program, Novavax Is currently facing a setback as it is experiencing a delay in the start of the U.S. leg of the late-stage study of NVX-CoV2373.

With two companies having already received conditional approvals for their vaccine candidates, Novavax is under pressure to expeditiously bring its coronavirus vaccine program to fruition.

Vaxart Inc. (VXRT) (+1940%)

This South San Francisco, California-based company was among the earliest to embark on a vaccine program against SARS-CoV-2. The company announced in late January the initiation of the program based on its proprietary oral vaccine platform, VAAST. The company selected a lead candidate, an oral recombinant vaccine, in late May.

In a bid to expedite its vaccine program, Vaxart appointed Andrei Floroiu, a biopharma industry veteran with experience in vaccine manufacturing, as its CEO in mid-June. The company filed an investigational new drug application for commencing the Phase 1 study in early August, and the application was cleared by the FDA in mid-September. About a month later, the company commenced the Phase 1 study of the investigational vaccine, codenamed VXA-CoV2-1.

"We look for interim follow-up safety updates at one month and two months post-dosing to be positive catalysts in 1Q21," HC Wainwright analyst Vernon Bernardino said in a note.

Cardiff Oncology Inc. (CRDF) (+1,563%)

Cardiff's shares had been flatlining since late-2018 up until the middle of 2020.

The company's lead clinical-stage candidate, onvansertib is a third-generation, oral and highly-selective adenosine triphosphate competitive inhibitor of the serine/threonine polo-like-kinase 1, which is over-expressed in multiple cancers including leukemias, lymphomas, and solid tumors. It is being evaluated in multiple Phase 2 studies for indications, including metastatic castration-resistant prostate cancer, colorectal cancer, and acute myeloid leukemia.

The shares began turning the corner after the company reported a $13.5 million equity investment in mid-June. It got another leg-up in mid-September following a presentation at the ESMO virtual conference, providing data on onvansertib in KRAS-mutant metastatic colorectal cancer. The rally has accelerated since then.

Genprex Inc. (GNPX) (+1,297%)

This gene therapy company received a shot in the arm from a mid-February licensing agreement with the University of Pittsburgh for a diabetes gene therapy that may have the potential to cure Type 1 and Type 2 diabetes. The stock, however, gave back much of the gain in about a month.

The company's immunogene therapy Reqorsa is being evaluated for non-small cell lung cancer.

Trillium Therapeutics Inc. (TRIL) (+1,068%)

The clinical stage immuno-oncology company has been broadly higher this year, although its shares have pulled back since early December.

The company is developing two pipeline assets: TTI-621 and TTI-622. TTI-621 is currently being evaluated in a phase 1 trial in patients with relapsed/refractory hematologic malignancies, while TTI-622 is being studied in a two-part, phase 1a/1b study in patients with advanced relapsed or refractory lymphoma or multiple myeloma.