Last week, the top-performing biotech stocks had something in common and by now you can probably guess exactly what that was.

You got it. They both delivered positive press releases regarding their respective coronavirus vaccine candidates.

Company (Symbol) Price Change During Week Ended Jun. 26, 2020  Market Cap
Vaxart (NASDAQ:VXRT) 219% $614 million
Inovio (NASDAQ:INO) 108% $4.7 billion

Data source: Yahoo! Finance.

1. Vaxart: Warp Speed selection

This clinical-stage biotech is working on vaccines delivered in the form of easy to swallow tablets. Vaxart stock exploded higher last week after the company's SARS-CoV-2 vaccine candidate was selected to receive extra assistance through the U.S. government's Operation Warp Speed program.

Coronavirus vaccine vials

Image source: Getty Images.

In May, the company selected an oral SARS-CoV-2 vaccine candidate for development and contracted with Emergent Biosolutions (NYSE:EBS) to produce the vaccine in sufficient quantities for pre-clinical and human-stage testing. Earlier this month, Emergent received a $628 million government order to manufacture the leading SARS-CoV-2 vaccine candidates. On June 26, Vaxart's SARS-CoV-2 vaccine candidate was selected to participate in a non-human primate challenge study organized and funded by Operation Warp Speed. 

If non-human primate studies with Vaxart's vaccine candidate are successful, the government will most likely pick up the tab for clinical trials too. As an easy-to-distribute tablet, Vaxart's candidate could have a leg up on competitors if it makes it that far. 

2. Inovio: Strong starter

This biotech has earned a reputation for quickly developing new DNA vaccine candidates to address infectious disease outbreaks. Since incorporating in 2001, the company has started clinical trials with DNA vaccines aimed at MERS, Ebola, and Zika virus before turning to the novel coronavirus immediately after its genetic sequence became available.

Inovio's stock price has been rising all year thanks to the company's rapid progress in early development stages. Earlier this month, Inovio began injecting people in a phase 2 clinical trial with their first dose of its SARS-CoV-2 candidate, INO-4800.

Last week the stock popped again after investors learned the U.S. Government would support the rapid manufacturing of a special DNA delivery device required to inject people with doses of INO-4800. By the end of the month, the company expects to report interim data from a phase 1 trial and positive results could send the stock higher still. 

Warning signs

While Operation Warp Speed aims to provide Americans with a safe, effective vaccine by January 21, it's important to realize that Vaxart can't possibly make this deadline if the company doesn't start testing real people immediately. Since the company's still in the middle of the preclinical-stage process, this doesn't seem likely.

Investors should know that Vaxart has been around since 2004, but it still doesn't have any late-stage clinical trial successes under its belt. The company's most advanced candidate beyond its SARS-CoV-2 program is a seasonal influenza vaccine. Vaxart still hasn't started a phase 3 study since producing underwhelming results during a phase 2 trial that wrapped up in 2017. While the stock could pop again if Vaxart's coronavirus vaccine candidate succeeds, its lackluster track record makes taking this bet a bad idea.

Inovio's another company that has started a lot of programs, but each one has fizzled out in mid-stage clinical trials. This is another stock best avoided until the company can show us some solid evidence that its vaccine technology actually works.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.