Jill Stein, the presumptive Green Party nominee, says there are many unanswered questions about medical vaccines.

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"As a medical doctor, there was a time where I looked very closely at those issues, and not all those issues were completely resolved," Stein said about potentially harmful side effects from vaccinations in an interview with The Washington Post conducted during the Democratic National Convention and published Friday.

"There were concerns among physicians about what the vaccination schedule meant, the toxic substances like mercury which used to be rampant in vaccines. There were real questions that needed to be addressed. I think some of them at least have been addressed. I don’t know if all of them have been addressed," the presidential contender added.

Stein also told the Washington Post many people remain skeptical of vaccines because of distrust of the Food and Drug Administration and government in general.

"I think there’s no question that vaccines have been absolutely critical in ridding us of the scourge of many diseases — smallpox, polio, etc. So vaccines are an invaluable medication," she said.

"Like any medication, they also should be — what shall we say — approved by a regulatory board that people can trust. And I think right now, that is the problem. That people do not trust a Food and Drug Administration, or even the CDC for that matter, where corporate influence and the pharmaceutical industry has a lot of influence.

"There is rampant distrust of our institutions of government right now," she continued. "The trust level for the presidency is somewhere around 15 percent. The strong confidence in Congress is somewhere around 4 percent, and the same is true of our regulatory agencies."

Stein has been critical before of corporate involvement in the vaccine approval process and the growing influence of lobbying in medical legislation.

Stein's comments sparked criticism from some on social media.

It's not the first time vaccines have become an issue during the 2016 election cycle.

Last year, Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE weighed in on vaccine controversy, saying that "the science is clear."

Two former Republican contenders, Gov. Chris Christie (N.J.) and Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (Ky.), had suggested last year that parents should be able to decide whether to vaccinate their children.

Christie later walked back those remarks, saying vaccines were an important public health measure.

Paul tried to put the controversy to rest by sharing a photo of him getting a booster shot.