Since outbreaks of the so-called swine flu first made headlines in April, Americans have nervously anticipated the virus' second coming in the fall, when traditional flu season begins. But while the first shipments of the vaccine for H1N1 are available, many people find themselves unable to get protection, even if they are in high-risk groups.
"My diabetes doctor does not have it, my private doctor does not have it," Maryland resident Kathy O'Grady, 38, said. "They tell me I need it and I can't get it."
Like other patients with chronic diseases, pregnant women are in danger of being left out initially because they can only receive the shot, not the nasal spray form delivered in many of the early shipments.
"That is frustrating because you know that was a missed opportunity and they made the effort and they came out and we wish we had the vaccine for them," said Dr. Ulder Tillman, a Montgomery County, Md., health officer.
O'Grady was among those turned away this week without a vaccine.
Susan Schwartz, a mother of two boys, Gary, 6 and Ryan, 3, had been waiting in line for four hours.
"I am still terrified that we are going to get to the front of the line and they are going to say, "So sorry, we just gave the last dose to the people in front of you,'" she said.
The problems are not limited to people like Schwartz in Rockville, Md., with doctors in states nationwide speaking of shortfalls in vaccine, many predicting that they will be forced to ration the supplies they have.
"It is a huge hassle because of all the extra communication required and work created by not having publicity and demand matched by supply," said Dr. Thomas Schwenk, chairman of family medicine at the University of Michigan in Ann Arbor. "We are having to make difficult decisions about who gets seasonal flu vaccine because of shortages and will have to do the same thing with H1N1."
Compounding the frustration is the fact that the Centers for Disease Control and Prevention had said last month that the vaccine -- originally not expected until November -- would be coming in sooner.
"We'll have on the order of 40-plus million doses of various types of vaccine by mid-October or late October," CDC Director Dr. Thomas Frieden said Sept. 25.
But delivery has fallen short of that, with production problems meaning that only 13 million are ready.
The shortfall has become a sore point on Capitol Hill, where a hearing was held Wednesday on the state of the vaccine.
Sen. John McCain, R-Ariz., was among the representatives pushing Health and Human Services Secretary Kathleen Sebelius for answers on when states will catch up from the vaccine shortfall.
The administration now promises 50 million doses by mid-November, and 150 million in December.
But some doctors declined to criticize the CDC for the shortfall.
"I think they have done the best they could do, under the circumstances," said Dr. Randy Wexler, an assistant professor of family medicine at Ohio State University. "The CDC has no control over vaccine production."
Government sources have laid the blame for vaccine delays at the feet of manufacturers, saying their estimates were overly optimistic.
Regardless of blame, however, some areas are being hit hard by swine flu.
St. Charles East High School in Chicago saw 45 percent of its 972 students absent this week because of flu-like symptoms.
And the flurry of illnesses has left some people wondering whether the vaccine will be too late to help.
"I think the answer to the question, will it be too late, for some people is, yes, will it be too late for the nation, I don't think so," said Dr. Greg Poland of the Mayo Clinic in Rochester, Minn.
ABC News' Lisa Stark contributed story.