President Joe Biden inadvertently gave his second in command a promotion in a gaffe as he touted his administration's feat in nearing 100 million COVID-19 vaccine doses.

His reference to Vice President Kamala Harris as "President Harris" during the press conference on Thursday comes after a poll last month showed one third of voters had doubts about his mental fitness for the country's top job.

During an address in the East Room of the White House, Biden said: "Now when President Harris and I took a virtual tour of a vaccination center in Arizona not long ago, one of the nurses on that, on that tour injecting people, giving vaccinations, said that each shot was like administering a dose of hope."

The president continued with his address in which Harris was standing behind him, without correcting himself. The transcript on the White House website carries his comments verbatim, but does correct Harris's position in brackets.

On February 22, a poll conducted by Business Insider and SurveyMonkey found that more than 33 percent of Americans were unhappy with Biden's mental sharpness.

U.S. President Joe Biden speaks as Vice President Kamala Harris looks on in the East Room of the White House on March 18, 2021 in Washington, DC. He mistakenly referred to his vice president as "President Harris."Drew Angerer/Getty Images

The survey, with a margin of error of three percent, found that 10.8 percent of Americans were "not so confident" while 22.8 percent said they were "not at all confident" in Biden's mental capabilities. However, it did find that 61 percent believed he was mentally capable for the job.

Facing criticism that Biden had not given a full press conference since taking office, a separate survey this month showed that half of Americans were concerned about Biden's physical and mental fitness.

In the Rasmussen poll conducted between March 3 and March 7, 50 percent of people answered that they were "not confident" that Biden was "physically and mentally up to the job." A majority (52 percent) said they were concerned he had not yet held a press conference. White House press secretary Jen Psaki has announced that Biden will give his first full media conference on March 25.

On the presidential campaign trail, former President Donald Trump made regular digs at his rival's mental acuity, calling him "sleepy Joe" and once telling Fox News that Biden was "mentally shot." Trump campaign Facebook ads carried messages like: "Joe Biden is clearly diminished."

However, Trump himself was regularly the centre of speculation about his own mental faculties and faced numerous calls for the 25th amendment to be invoked, including during the presidential transition. In April 2019, the World Mental Health Coalition produced a mental health analysis of Trump and concluded he was unfit for any job.

Meanwhile a CNBC/Change Research poll conducted in September 2020 in six swing states found a majority of respondents (51 percent) believed Trump was mentally unfit to be president. This was one percent less than those who responded that Biden was mentally unfit for the job (52 percent).

Biden has made other gaffes which have set conservative tongues wagging. In December, he mistakenly referred to Harris as "president-elect," also during a speech on COVID-19 vaccines.

Harris, too, is not immune to a slip of the tongue. Last September, she accidentally dubbed herself commander-in-chief during a virtual roundtable last September in which she mistakenly referred to "a Harris administration."

Although unintentionally uttered, this prospect is not beyond the bounds of possibility if recent odds being offered by bookmakers bear out in reality. Earlier this month, Ladbrokes named Harris as the favorite to win the 2024 presidential election, with 22 percent implied probability—ahead of both Biden and Trump, Reuters reported.

Newsweek has contacted the White House for comment.