Two years ago, the nation was beset by a severe shortage of flu shots, with huge lines at clinics and many people going without. This year, it looks as if there might be a glut.
Yet, somewhat perversely, because of distribution delays earlier in the season, this year's abundant supply has not meant that everyone who wanted a flu shot has received one.
The situation underscores the uncertain nature of the nation's supply system for flu vaccine, a risky and volatile business with thin profits, in which the federal government has a limited role.
While experts say an excess is better than a shortage, too large a surplus could hamper the government's goal of steadily increasing the production and use of flu vaccines. Because makers, distributors, doctors and health departments lose money from vaccine they cannot sell to patients, they might be discouraged from making or ordering as much in coming years - potentially leading to future shortages.
And there is little opportunity to stockpile for lean years, because flu vaccine is good for only a single season. The composition must be changed each year to match the strains of influenza virus in circulation.
Some manufacturers dropped out of the flu vaccine business in the past, in part because they could not sell all they made. That set the stage for the shortage in 2004, when there were only two major suppliers, and one of them, Chiron, had to suspend production because of sanitary problems.
This time around, experts say millions of doses might go unused, in part because production is at an all-time high. The four vaccine suppliers are expected to make as many as 110 million to 115 million doses, with more than 100 million having been delivered. The most vaccine ever distributed in any previous year was 83 million doses.
In Ohio, for instance, about 100,000 of the roughly 250,000 doses that the state health department acquired for distribution to local health offices remain unused, said Michael Stefanak, the Mahoning County health commissioner. His department, as of early this week, still had 2,700 of the 5,500 doses it received from the state.
If drumming up demand for so much vaccine was going to be a challenge, the task was made harder by manufacturing and distribution snags that caused shortages in some places in September, October and early November - the months when most people are accustomed to getting vaccinated.
When millions of doses finally got delivered in late November or this month, it was too much, too late, many health officials say. Holiday distractions make it difficult to get people to come for immunization in December. Also, the flu season has been relatively mild this year, making a flu shot seem less urgent.
"There's so many people out there that need it and we know didn't get it yet," said Dr. Henry H. Bernstein, a pediatrician at Dartmouth Medical School and a member of the infectious diseases committee of the American Academy of Pediatrics.
Under current medical guidelines, a yearly vaccination is recommended for 218 million of the nation's population of 300 million, a number that has been steadily increasing. But many people in the recommended groups decline to get vaccinated, in part because they do not consider the flu a serious enough disease, health officials say.
But changing an entrenched mind-set can be difficult. That is why even though many more people could still end up getting vaccinated this season - especially if a severe outbreak of flu occurs soon - some industry and public health officials are assuming there will be leftovers.
Indeed, the day after the national vaccination week ended, Henry Schein Inc., a major vaccine distributor, said its profits would suffer because it would sell only 9 million doses, 4.5 million fewer than expected. Steven Paladino, executive vice president, said that by the time the company received several million doses, its customers no longer wanted them.
Schein got those flu shots from a GlaxoSmithKline subsidiary in Canada that was entering the U.S. market for the first time. It did not receive Food and Drug Administration approval to sell its vaccine until early October.
Another reason for the early-season delay was that one of the virus strains in this year's vaccine grew more slowly than expected in the millions of chicken eggs used to multiply the viruses. Because of that, Sanofi-Aventis, the largest supplier of the vaccine, did not begin shipping its first lots until Aug. 23, about three weeks later than usual, Patricia Tomsky, a spokeswoman, said.
The government has been trying to coax more vaccine supply into the market, in part by speeding approvals, to lessen the risk of a shortage such as the one in 2004. Having greater manufacturing capacity also would make it easier to produce enough vaccine if a pandemic should arise.
The authorities also say more people should be immunized to reduce the estimated 36,000 deaths and 200,000 hospitalizations a year from influenza. This year for the first time, vaccination was recommended for children from age 2 until their fifth birthday, having been previously recommended only for children 6 months to 23 months old.
This year there are three manufacturers of flu shots: Sanofi-Aventis, which has shipped 50 million doses; Novartis, which acquired Chiron and has shipped at least 31 million doses; and GlaxoSmithKline, which has shipped 24 million. MedImmune, which makes a nasal spray vaccine, has sold 2.5 million doses.
The companies say a single year of surplus will not deter them from long-term commitments to increase output, but some experts say they are worried.
"I'm concerned that we'll throw away 20 million doses," said Dr. L.J. Tan, the co-chairman of the National Influenza Vaccine Summit, a group of 140 companies, nonprofit groups and public agencies. "If we keep on doing that, I can guarantee that the following seasons we won't have 115 million doses anymore."
Manufacturers lose money if they cannot sell all they make. But once they sell it, they generally do not accept returns. So, some health care providers say they might order less next year if they are left with unused vaccine this year.