The September 9-15 tracking poll showed that 42 percent of likely voters supported Clinton while 38 percent backed Trump.
Clinton, who has mostly led Trump in the poll since the Democratic and Republican national conventions ended in July, regained the advantage this week after her lead briefly faded in late August.
Clinton has an advantage among minorities, women, people who make more than $75,000 a year, and those with moderate political leanings. Trump has an advantage with whites, men, avid churchgoers, and people who are nearing retirement age.
Overall, Americans appear to be relatively uninspired by their choices for president with less than eight weeks to go before the election. One out of every five likely voters said they do not support Clinton or Trump for president.
In comparison, about one out of every 10 likely voters wouldn't support Obama or Republican challenger Mitt Romney at a similar point in the 2012 presidential campaign.
Respondents took the survey after video surfaced of Clinton nearly collapsing at a Sept. 11 memorial in New York on Sunday.
Her campaign later said she had a non-contagious, bacterial form of pneumonia.
The video sparked a renewed discussion about the health of both candidates. Trump, 70, would be the oldest president to take office, while Clinton, 68, would be the second oldest.
Clinton and Trump candidates have since released details of their personal health. Clinton's doctors said her physical exam was normal, apart from the pneumonia, and that she was in excellent mental condition. Trump released a note from his doctor saying that he was in "excellent physical health."
Americans do not appear to be overly concerned with the health of either candidate. According to a separate Reuters/Ipsos poll conducted this week, most American adults said the issue would make "no difference" to how they voted.
A negligible percentage of Clinton supporters said concerns about her health made them "less likely" to vote for her.
Clinton led all candidates in a four-way poll of likely voters that included Gary Johnson of the Libertarian Party and Jill Stein of the Green Party. Seven percent of respondents supported Johnson, and 2 percent backed Stein.
The Reuters/Ipsos poll is conducted online in English in all 50 states. The latest horserace poll surveyed 1,579 likely voters over the past week. The separate poll on the candidates' health surveyed 1,179 American adults from Sept. 12-16. Both polls had a credibility interval, a measure of accuracy, of 3 percentage points.
National polls have produced varying measurements of support during the 2016 campaign for Clinton and Trump. The differences are partly due to the fact that some polls, like Reuters/Ipsos, try to include only likely voters, while others include all registered voters. The Reuters/Ipsos tracking poll gathers responses every day and reports results twice a week, so it often detects trends in sentiment before most other polls.
Polling aggregators, which calculate averages of major polls, have shown that Clinton's lead over Trump has been shrinking this month. The most recent individual polls put
Clinton's advantage at 1 or 2 percentage points.
