President Trump's infection with COVID-19 is the epitome of an October surprise that can alter the path of an election, but how it will tilt the playing field is an unknown so far.

Betting markets and polls give the advantage to former Vice President Joe Biden at the moment, but skeptics point out that UK Prime Minister Boris Johnson, a frequent Trump ally, saw a boost in ratings after he contracted the virus earlier this year.

“Markets appear to be increasingly pricing Joe Biden in as the favorite, and this news may not change that, but Trump could gain support from a quick recovery as UK Prime Minister Boris Johnson did during his battle with COVID-19,” said Jeff Buchbinder, equity strategist for LPL Financial.

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Betting markets have been moving in favor of Biden following the president’s positive coronavirus test, with a collection of figures compiled by RealClear Politics showing the former vice president with a 61% chance of winning, up from 54.5% on Wednesday.

Additionally, Biden holds a 7.2-point lead in polls nationally and is up an average of 3.5 points in key battleground states, according to RealClear Politics. The polls do not yet reflect Trump's contraction of the coronavirus.

However, if public sentiment responds in the same way that it did in Boris Johnson's case in the U.K., polls could actually tighten during the election run-up.

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Johnson saw his support rise sharply in the more than two weeks after he came down with the virus. They have since fallen sharply as his government struggled to contain the spread of the disease.

Trump’s approval rating, meanwhile, recently reached 46%, up from 38% three months prior, according to Gallup. That's just 3 points below his high-water mark of 49%, reached several times earlier this year.

The president’s critics, however, are likely to ramp up attacks over what they say is more evidence of him not taking the virus seriously. Trump has often refused to wear a mask while in public, arguing that people he is in close contact with are tested.

Trump was taken to Walter Reed National Military Medical Center on Friday afternoon, and his diagnosis has wide-ranging implications for his bid for a second term, forcing him off the campaign trail and lurching the upcoming debates with Biden into a state of uncertainty.

Being out of the public view could make it harder for the president to tilt public sentiment back into his favor ahead of Election Day.