The biggest frustration facing many doctors is the dearth of swine flu vaccine for their patients. But a small cadre of outliers remains skeptical about the government’s unprecedented immunization campaign, citing doubts about the risks presented by the H1N1 virus or the safety of the vaccine.

“My feeling is that this is all being over-hyped,” said Laurence J. Murphy, a pediatrician in Burke, Va., who will not inoculate his patients. “Most people who get this virus do beautifully. I believe the vaccine hasn’t been tested enough. I just think the benefit of it at this point is not outweighed by the possible risk.”

Such contrarian voices, through the megaphone of cable news or in the quiet of exam rooms, have forced federal health officials to play defense as well as offense in their campaign to encourage immunization.

Officials repeatedly have stressed that while no vaccine is completely safe, there is no reason to believe the swine flu immunization would pose any unusual risks, and so far no problems have emerged.

Public health leaders are at a loss to explain the skeptical minority, except to say that it mirrors the chronically low percentage of health care workers who get the seasonal flu vaccine every year. Officials worry that these doubters could have a disproportionate influence in an already frustrating and confusing situation, and stress that the studies conducted so far and the intensive monitoring under way indicate that the vaccine is as safe as any flu vaccine.