Johnson & Johnson announced Monday night that it is pausing its coronavirus vaccine trial after one of its study participants came down with an "unexplained illness." It's not clear if the patient received the vaccine itself or a placebo shot.

The pharmaceutical company is currently in the Phase 3 stage of its trials, in a study the company calls ENSEMBLE.

"Following our guidelines, the participant’s illness is being reviewed and evaluated by the ENSEMBLE independent Data Safety Monitoring Board (DSMB) as well as our internal clinical and safety physicians," the company said in a statement.

The company added that "adverse events -- illnesses, accidents, etc. -- even those that are serious, are an expected part of any clinical study."

Johnson & Johnson said it is working to gather more facts about the situation before it shares more information with the public.

According to the World Health Organization, there are 10 vaccines currently in late-stage Phase 3 trials.

NFL owners will meet virtually Tuesday to weigh their options on how to schedule the rest of the season, as several games have been postponed due to COVID-19 positive tests.

The NFL Network reported owners are mulling shuffling bye weeks and games to make the league's standard 17-week slate work.

Although owners prefer not to go to an 18-week schedule, it may be under consideration, according to reports.

"In a Week 18 scenario, the league would likely only play games if they had a significant impact on the playoffs," the NFL Network reported. "And if such a postponement scenario plays out for the same teams multiple times, there's even a chance for a Week 19."

The scientific journal "The Lancet" published new information Monday about the Nevada man who contracted COVID-19 twice, and warned that even those who have recovered from the virus need to take precautions.

The 25-year-old patient had no known immune disorders or underlying conditions when he tested positive for the virus in April and again in June, according to the study. The patient was hospitalized during his second infection with severe symptoms, including dizziness, cough, nausea, and diarrhea.

He was released from the hospital and has since recovered, according to the journal.

Mark Pandori of the Nevada State Public Health Laboratory, the lead author of the study, said in a statement that while more research is needed, the data indicates that patients who recovered from an infection should still heed health warnings, such as avoiding crowds, wearing a mask and washing their hands.

"There are still many unknowns about SARS-CoV-2 infections and the immune system's response, but our findings signal that a previous SARS-CoV-2 infection may not necessarily protect against future infection," he said in a statement.

Experts say that people who recover from COVID-19 will likely have some form of immunity, but it's not clear if everyone develops immunity, or how long that immunity might last.

Reinfections are rare. The Nevada man is the only reported case in the U.S. so far of a patient contracting the virus twice. Four other cases of reinfection have been reported in Belgium, the Netherlands, Hong Kong, and Ecuador, according to the journal.

"So far, we've only seen a handful of reinfection cases, but that doesn't mean there aren't more, especially as many cases of COVID-19 are asymptomatic," Pandori said.

-ABC News' Sony Salzman contributed to this report.

Dr. Anthony Fauci again called on the Trump campaign to take down a recent political ad that used previous statements that he says were taken out of context, warning that the future use of his interviews and statements could "backfire" on the campaign.

Fauci appeared on CNN Monday and reiterated to Jake Tapper that he didn't give permission for his comments in a March Fox News interview, in which he complimented the overall efforts of federal public health officials, to be used in an ad in which it sounded like he was praising Trump personally.

"It's so clear that I'm not a political person. I have never either directly or indirectly endorsed a candidate," said Fauci, the director of the National Institute of Allergy and Infectious Diseases.

Despite Fauci's objections, Trump and the campaign officials said Sunday they would continue to run the ad.

When asked how he would react if the campaign ran more ads featuring his statements, Fauci said it would be "terrible" and "outrageous."

"That might actually come back to backfire on them; I hope they don't do that because that's, that would be kind of playing a game that we don't want to play," he said. "So I hope they reconsider that, if in fact they are indeed considering doing that."

Later Monday evening, Democratic presidential candidate Joe Biden tweeted a tongue-in-cheek video that cut together clips of Trump's speeches and interviews to make it look Trump apologized to Fauci.

"Donald Trump is running TV ads taking Dr. Fauci out of context and without his permission. So, here's a message from the President in his own words," Biden tweeted.

Biden later slammed Trump for going against Fauci's wishes during a campaign stop in Cincinnati, Ohio, saying, "Look, they use the ad knowing it's a lie. Can you remember any other major mainstream presidential candidate ever doing something like these things?"