For one, it really wouldn’t be that much of a surprise. That is, Trump has been promising a vaccine for months. His list of campaign promises included a vow to produce a COVID-19 vaccine by the end of the year and “return to normal” in 2021. In many ways, a vaccine is already priced into the market.
It may be that there are some swing voters who are skeptical that a vaccine will appear this year but would be swayed toward Trump if he defeated that skepticism, but likely not many. Indeed, many voters say that a quickly produced vaccine is one that would make them more skeptical, and they’d be unlikely to have it administered. Trump’s relentless politicization of the vaccine process has already limited the impact an October vaccine would have.
Second, despite a very news-heavy year, this has been a remarkably stable presidential contest. Unlike in 2016, when Hillary Clinton’s lead over Trump grew to double digits and shrank to nothing multiple times throughout the year, Biden has held a very steady lead over Trump since he clinched the Democratic nomination. The major party conventions, which usually move the needle back and forth, barely budged polls this cycle. Like most presidential re-election campaigns, this one seems to be turning on people’s evaluations of the president’s first term, and there aren’t many people undecided on that or even with soft opinions on it.