WASHINGTON – The director of the Centers for Disease Control and Prevention predicted Wednesday that most of the American public will not have access to a vaccine against the novel coronavirus until late spring or summer of next year – prompting a public rebuke from President Donald Trump, who declared the CDC chief was wrong.

At a Senate hearing on the government’s response to the pandemic, CDC director Robert Redfield adhered to Trump’s oft-stated contention that a safe and effective vaccine will become available in November or December – perhaps just before the presidential election seven weeks away.

But Redfield said the vaccine will be provided first to people most vulnerable to COVID-19, the disease caused by the virus, and supplies will increase over time, with Americans who are lower priority for the protection offered the shot more gradually. For it to be “fully available to the American public, so we begin to take advantage of vaccine to get back to our regular life,” he said, “I think we are probably looking at late second quarter, third quarter 2021.”

Hours later, Trump sought to knock down Redfield’s predicted timeline from the White House press briefing room, saying at a news conference, “I think he made a mistake when he said that. … We’re ready to distribute immediately to a vast section of the country.”

The president said that, when he heard what the CDC director had told senators, he called him directly. Trump said Redfield “didn’t tell me that,” though the president declined to disclose how Redfield replied.

“It was an incorrect statement. … We are ready at a much faster level than he said,” Trump said, reiterating a recent talking point that a vaccine could be ready to begin administering as early as mid-October.

Multiple experts – including top scientists leading the vaccine effort – have said it is unlikely a vaccine will be available by then.

Speaking alongside the president, Scott Atlas, a recent addition to the White House’s coronavirus advisers, noted that the administration Wednesday circulated a vaccine distribution strategy to states and others. Atlas said the plan anticipates that “no later than January, all the top-priority people will be able to receive the vaccine,” with other Americans receiving it starting soon after.

The CDC director issued his prediction and received the presidential drubbing the same day that Democratic presidential nominee Joe Biden raised questions about the safety of a coronavirus vaccine approved during Trump’s tenure, warning something so complex and vital to the public’s well-being takes time.

“Scientific breakthroughs don’t care about calendars any more than the virus does,” the former vice president said.

Redfield said that though any individual vaccinated should benefit from a vaccine, the progressive widening of its availability means there will be a time lag between when a vaccine is approved and when it could have a measurable effect in controlling the pandemic. That might be six to nine months after the day it is approved by federal drug regulators, Redfield predicted.

Redfield said that lag between when a vaccine is approved and when the public can get it reinforces the importance of safety measures, such as keeping a proper distance, washing hands and wearing masks.

“I might even go so far as to say that this face mask is more guaranteed to protect me against COVID than when I take a COVID vaccine,” Redfield said, because the vaccine is unlikely to produce the desired immune response in everyone who gets it.

But Trump at his briefing continued to cast doubt on the value of masks, saying, “The mask is a mixed bag.”

Redfield’s comments were the most detailed time frame outlined so far by the leader of the government’s main public health agency. They are consistent with the perspective of Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, who said in a recent interview with CNN that relatively small amounts of vaccine will be available at first.

“It won’t be until we get into 2021 that you’ll have hundreds of millions of doses, and just the logistics, constraints in vaccinating large numbers of people,” Fauci said. “It’s going to take months to get enough people vaccinated to have an umbrella of immunity over the community so that you don’t have to worry about easy transmission.”

Redfield’s forecast came as Trump has latched onto the prospect of a vaccine as crucial to his prospects for a second term, with low approval ratings among voters for his handling of the worst public health crisis that the country and world have confronted in a century.

A vaccine also is widely regarded as a pivot point for Americans to be unfettered from the constraints the pandemic has imposed on daily life – from recreation such as concerts and movie theaters to workplaces that remain shuttered.

A race is underway internationally among pharmaceutical makers to develop vaccines that are safe and effective against the virus, which has infected nearly 6.6 million people in the United States and killed almost 200,000. Developing a vaccine typically takes years, but researchers are working with unprecedented speed. U.S. researchers in January established the goal of a world-record pace of developing an inoculation against the coronavirus within a year to 18 months.

Now, three experimental vaccines have entered the final stage of testing in the United States – giving each one to thousands of people to check effectiveness and safety – before submission for federal approval. A debate is raging over whether the Food and Drug Administration should hasten a vaccine’s availability by employing emergency authority it has before going through the process of a formal approval.

The CDC told states this month they should be ready to receive a coronavirus vaccine as early as Nov. 1 – two days before the election – prompting allegations from critics that the date was politically motivated. Sen. Patty Murray, D-Wash., the subcommittee’s senior Democrat, accused the administration of “rampant political interference in scientific decision-making.”

Redfield pressed back against such suggestions during an appearance Wednesday before a Senate subcommittee with jurisdiction over the Department of Health and Human Services, of which the CDC is a part. He said the advice to states was based on the pace of the science, not any electoral considerations. And he said his agency was eager to avoid a repetition of a problem that emerged during a pandemic of the H1N1 virus in 2009, when a vaccine became available and states were not ready to receive and distribute it.

“We don’t want to repeat that hiccup,” Redfield told senators.

He also said the government does not have an estimated $6 billion it needs for the distribution of a coronavirus vaccine. Such funds were proposed in pandemic relief legislation that Congress has not adopted, among partisan disputes over how much more aide the government should provide for laid-off workers and a variety of other purposes.

Providing that money, Redfield said, “is as urgent as getting these manufacturing facilities up.”

Biden’s remarks Wednesday show how the pandemic has increasingly become a focal point for both candidates in the final weeks of the race. Biden campaign advisers have regarded the election as a referendum on Trump and his handling of the pandemic. The campaign continues to hold events and run advertisements squarely on this theme.

The former vice president’s comments, extending suspicions Biden has expressed in recent weeks, highlight the extraordinary roughness of this presidential contest. In past election cycles, calling into question whether an incumbent might risk deliberate harm to Americans to forward his political ambitions was not the norm.

But speaking in Wilmington, Del., Biden expressed reservations about whether a covid-19 vaccine approved by the Trump administration would be safe, casting doubt on the incumbent’s willingness to put the health of Americans before politics.

“I trust vaccines. I trust scientists. But I don’t trust Donald Trump,” Biden said. “And at this point, the American people can’t, either.”

Biden raised the possibility of Trump pressuring his administration’s health officials to sign off on a vaccine in which scientists do not yet have full confidence in order to gain an election advantage. The Democratic nominee expressed skepticism about the CDC and FDA, as well as the president.

The former vice president essentially echoed Redfield’s point that vaccinating the nation will happen gradually. “It’s not going to happen overnight,” Biden said. “Once we have it, it’s going to take months to distribute.”

If a vaccine is swiftly approved, it could upend the campaign, and both sides are increasingly bracing for how to deal with the political uncertainty of the coming weeks. Still, experts have questioned whether it is realistic for one to become available before the election.

Biden made his remarks after receiving a briefing Wednesday about the quest for a vaccine from scientific, public health and health policy experts. Creating the drug is only “part of the battle,” said Biden, who likened effective distribution to a complex military operation. He said a vaccine should be free and that priority should go to those who need it most – and that includes Black and Hispanic communities.

The Democrat’s view about the possibility of a vaccine has become a point of contention in the campaign, with Trump accusing Biden and his running mate, Sen. Kamala Harris, D-Calif., of spreading “anti-vaccine rhetoric.”

Biden said he would have no problems endorsing a vaccine – provided it met certain criteria. If the current administration allows a vaccine to be distributed, Biden said, “who will validate it was driven by science? What groups of scientists?”

He added that Americans must be confident “distribution will be safe and cost-free” with a plan for doing so “without a hint of favoritism.”

Biden also lambasted Trump for not aggressively encouraging mask-wearing and alleging that waiters do not like to wear them. The Democrat defended his own calls for a national mask mandate, saying he would seek to implement one by working with governors but that he was not yet completely sure what legal authority he would have to deploy such a rule.