US presidential rivals George W Bush and John Kerry have launched new attacks on each other about the shortage of flu vaccinations.
Across the country, queues of Americans are waiting for their jabs
A new Democrat TV ad calls the shortage a "George Bush mess" and Mr Kerry told a Florida rally that the president's health care plan was "don't get sick".
Mr Bush hit back, accusing Mr Kerry of using scare tactics as he tried to allay any fears among voters.
Two weeks before election day, it is still a race that is too close to call.
Both presidential candidates have been in Florida - seen as a key state to win and one which has a large proportion of elderly voters.
The shortage of flu vaccine came about after British regulators shut down shipments from a UK production line, cutting the US supply of flu shots almost in half.
The factory was to have provided the US with 48m doses.
Health and Human Services Secretary Tommy Thompson said on Tuesday that vaccine manufacturer Aventis Pasteur would be able to produce an extra 2.6 million doses.
Mr Bush said the US was doing everything it could to overcome the problems he said were caused by a "manufacturing defect".
"I know there are some here who are worried about the flu season," Mr Bush said at a rally in St Petersburg, Florida.
"I want to assure them that our government is doing everything possible to help older Americans and children to get their shots," he said, adding that millions of vaccine doses were available for those who needed them the most, and more were being rushed in.
But the Kerry campaign says the Bush administration was warned about the shortage of flu vaccines three years ago, and did not act.
"Instead of fixing the problem, production of the vaccine was sent to a factory overseas [and] the vaccines were contaminated," the ad says.
Mr Kerry also attacked the handling of the problem when he was in Florida on Monday, berating his opponent's health care plan as "don't get sick, just pray, stand up and hope, wait, whatever."
US health authorities, who had been trying to promote jab take-up among all Americans, are now asking only those at greatest risk to request vaccination, in order to prolong supplies.
Queues have been forming at pharmacies as Americans scramble to get vaccinated.
About 1,000 high-risk residents of Bloomfield, New Jersey, entered a lottery to be used to decide who will get the town's remaining 300 jabs.
The return to domestic issues on the campaign trail came on a day when new polls gave conflicting opinions on Mr Bush's standing.
A survey for the Washington Post put Mr Bush's job approval rating at 54% and the newspaper reported that, in the modern era, all presidents with approval ratings above 50% have won their re-election bids.
But another poll released on Tuesday for the New York Times and CBS News said Mr Bush's rating was at 44%, which the newspaper called "a dangerously low number for an incumbent president, and one of the lowest of his tenure".
National polls continue to show opinion effectively split between Mr Bush and Mr Kerry, with Mr Bush leading in some surveys.
The Washington Post said though its survey gave Mr Bush a slender lead, Mr Kerry was ahead in 13 swing states by 50% to 46%.