Donald Trump and Joe Biden have been trading insults over each other’s position on a vaccine for Covid-19.
President Trump again hinted that a vaccine might be available before the November presidential election and accused his Democratic rivals of “reckless anti-vaccine rhetoric”.
Mr Biden expressed scepticism that Mr Trump would listen to the scientists and implement a transparent process.
The US has six million cases of coronavirus, the highest in the world.
The virus has also claimed nearly 190,000 lives and fuelled a major recession, double-digit unemployment and sagging consumer confidence.
Last week it emerged the US Centers for Disease Control and Prevention (CDC) had urged states to consider “waiving requirements” in order to be able distribute a vaccine by 1 November – two days before the 3 November election.
No vaccine has yet completed clinical trials, leading some scientists to fear politics rather than health and safety is driving the push for a vaccine.