In a video message released on December 11, 2020, US President Donald Trump claimed the nation had achieved a "medical miracle".
The first COVID-19 vaccine had just been approved in the US, where the pandemic had already killed more than 292,000 people.
The announcement marked a critical milestone for Operation Warp Speed — a program launched by Mr Trump to accelerate the development, manufacture and distribution of COVID-19 vaccines and therapeutics.
"We have delivered a safe and effective vaccine in just nine months," he said in the recorded message.
"This is one of the greatest scientific accomplishments in history."
Five years on, Mr Trump has credited his program with saving tens of millions of lives — something he described as "one of the greatest achievements ever" in late August.
But as the president continues to laud his own pandemic response, a political flashpoint could be developing over the anti-vaccination stance of his hand-picked health secretary, Robert F Kennedy Jr.
Sparring with the Senate
In a packed room inside a Washington DC office building last week, Mr Kennedy faced questions from a panel of politicians.
A storm had been developing around his department for months, and the Senate Finance Committee wanted answers.
The Centers for Disease Control and Prevention (CDC) director had been ousted after just 29 days in the role, the Food and Drug Administration (FDA) had severely limited access to COVID-19 vaccines, and $US500 million ($752 million) in research into mRNA vaccines had been abandoned, all under Mr Kennedy's watch.
Mr Kennedy had also broken a vow not to change the CDC's advisory committee on immunisation practices, firing all 17 serving members and naming several vaccine critics among their eight replacements.
Republicans who once supported Mr Kennedy's appointment were now sounding the alarm over the state of the Department of Health and Human Services.
The panel spent 3 hours attempting to needle down Mr Kennedy's opinions on vaccines and the nation's pandemic response, exposing fault lines between Mr Trump and the health secretary.
At times, the health secretary said he could not put a number on how many Americans died from COVID-19, nor confirm whether vaccines prevented any deaths.
Republican senator Bill Cassidy used a pointed line of questioning to probe Mr Kennedy on the pandemic response, asking whether he agreed the president deserved a Nobel Prize for spearheading Operation Warp Speed.
"Absolutely, senator," Mr Kennedy said.
The reply exposed a contradiction for Mr Cassidy, who noted the health secretary's long history of vaccine scepticism.
"It just seems inconsistent that you would agree with me that the president deserves tremendous amount of credit for this," he observed.
The health secretary has been a known critic of vaccines for years, but his elevation to government has put him in a rare position to turn his words into action.
Before taking a tilt at politics, Mr Kennedy had chaired the influential anti-vaccine organisation Children's Health Defense.
In that role, he frequently disparaged vaccines and promoted debunked claims, including that they cause autism and illnesses.
While Mr Kennedy has frequently said Americans should not take medical advice from him, the influence of his words can be seen trickling across the country.
Florida hands vaccine sceptics huge win
In Florida, the health secretary's political allies have unveiled plans to lift vaccine mandates for schoolchildren.
State Surgeon General Joseph Ladapo, who announced the decision, called the vaccine mandates "immoral" and said they dripped with "disdain and slavery".
The state would lift requirements for students to be vaccinated for hepatitis B, chickenpox, Hib influenza and pneumococcal diseases such as meningitis, its health department said.
Vaccines for measles, polio, diphtheria, pertussis, mumps and tetanus would still be required.
The state health department said on September 3 the changes likely would not take effect for 90 days.
The plan was endorsed by Mr Kennedy's administrator for Medicare and Medicaid Services and former television presenter, Dr Mehmet Oz.
In an interview with Fox News, Dr Oz said doctors "shouldn't feel pressure from the government to decide what to do with the vaccination schedule".
"I would definitely not have mandates for vaccinations," he said.
Margie Danchin, a professor of paediatrics and vaccinology at the University of Melbourne and Murdoch Children's Research Institute, said Mr Kennedy's messaging around vaccines would have made room for Florida to pursue changes to vaccination requirements.
"It's very powerful language and it undermines trust and confidence in vaccines," she said.
"It absolutely has led to Florida withdrawing mandates for vaccines in schools."
The policy change came as the state's governor, Ron DeSantis, announced the launch of a state-based Make America Healthy Again commission, which aligned with Mr Kennedy's goals, such as promoting informed consent and parental rights.
Blue states attempt to insulate
As the fight against vaccine science progresses, some Democrat-run states have begun considering ways to protect vaccine access for their residents.
The governors of Washington, Oregon and California announced on September 3 they had struck an alliance to establish their own recommendations for who should receive vaccines.
They plan to coordinate their vaccine recommendations and immunisation plans based on science-based evidence from respected national medical organisations, according to a joint statement.
Other states, such as Pennsylvania, Colorado and Massachusetts, have moved to protect the availability and accessibility of vaccines like COVID-19 boosters.
Dr Danchin said blue states would also need to do more to restore trust in vaccines to counteract Mr Kennedy's messaging.
"They're going to have to work very hard at a local and community level, because I think the dissension is really undermining trust," she said.
Treading on thin political ice
The spread of vaccine hesitancy could be the first marker of a new political crisis swirling for the Trump administration, onlookers say.
Routine childhood vaccination rates are already falling, and fewer Americans say it is important for children to be vaccinated.
It means once-eradicated infectious diseases returning is almost a certainty as childhood vaccine rates fall, Dr Danchin said.
"You can absolutely guarantee that America is going to see an upswing in vaccine-preventable diseases for children," she said.
"We're going to see more vulnerable populations getting sick, and kids dying and elderly people dying, and immunocompromised people dying. We only have to look at Africa and the Asia-Pacific region and see what those countries are grappling with right now."
Already, the US is experiencing its largest outbreak of measles in 25 years, which has resulted in three deaths and more than 1,450 cases across 42 states, as a result of declining immunisation coverage in some communities.
About 92 per cent of the people infected were unvaccinated, or their vaccination status was unknown.
The issue presents a tightrope for the Trump administration, Harry Melkonian, an honorary associate at the United States Studies Centre, said.
Senior aides have credited Mr Kennedy's loyalists with cinching the popular vote for Mr Trump, and they could prove crucial again at the midterms next year.
But the majority of Americans still believe parents should be required to vaccinate their children against preventable diseases to attend school, and several Republicans, who were doctors before they entered politics, have made clear they will not compromise on medical science.
Dr Melkonian added that Mr Trump "does not want to alienate Robert Kennedy" while attempting to appease other Republican factions.
"That could be disastrous because probably in lots of electorates, Robert Kennedy's followers could very well be a winning margin," he said.
"When he was a candidate, he had a measurable percentage of supporters."
So far, the president has publicly aligned with his health secretary on the Make America Healthy Again agenda.
"It's not your standard talk, I would say that, and that has to do with medical and vaccines," Mr Trump told reporters after Mr Kennedy's Senate hearing appearance.
"But if you look at what's going on in the world with health, and look at this country also with regard to health, I like the fact that he's different."
Still, Mr Trump's support appears to have a limit, with the president stopping short of endorsing Florida's plan to lift vaccine mandates.
"Look, you have some vaccines that are so amazing — the polio vaccine, I happen to think, is amazing," he said in the Oval Office, adding that "you have to be very careful" when changing vaccine requirements.
"I think those vaccines should be used, otherwise some people are going to catch it, and they endanger other people."
Scott Gottlieb, the FDA commissioner during Mr Trump's first administration, told CNBC that Mr Kennedy's vaccine scepticism was at odds with what he knew to be the president's opinions.
"The president always affirmed the importance of the paediatric schedule," Dr Gottlieb said.
"I think Kennedy is at odds with both the president's ethos around these things, and certainly the Senate."
Dr Melkonian said despite the health secretary encountering resistance from all sides, it as clear he was "going to stay" in government.
"Trump has got to find some way to make this work," he said.
"And now that Robert Kennedy is stepping on toes, he's got to be a little more guarded."
