"Mrs Trump continues to feel better every day following her recovery from COVID-19, but with a lingering cough, and out of an abundance of caution, she will not be traveling today," Stephanie Grisham said in a statement.
The first lady's appearance with Trump in Erie, Pennsylvania, was to have been her first at a campaign rally in more than a year.
Trump, the first lady and their teenage son Barron all tested positive for Covid-19 earlier this month.
The president was hospitalised for three nights and later said he'd risked death, but now appears to have recovered. The first lady's case was described as having been considerably milder.
However, last week she revealed details of the illness in an unusual statement entitled "My Personal Experience with Covid-19" in which she said the aftermath had been difficult.
"It seemed to be a roller coaster of symptoms in the days after. I experienced body aches, a cough and headaches, and felt extremely tired most of the time," she said.
Melania Trump said that in contrast to the president, who took an intense therapy of experimental drugs and received oxygen to assist his breathing, "I chose to go a more natural route in terms of medicine, opting more for vitamins and healthy food."
It was only in that statement that she revealed that Barron, 14, had also become infected, although he has since recovered.
Trump has been holding at least one election rally a day in the last two weeks before Election Day but he has not appeared on stage with his wife since 2019.
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