People who do not believe in vaccinating children have never had much sway over Leslie Wygant Arndt. She has studied the vaccine debate, she said, and came out in favor of having her 10-month-old daughter inoculated against childhood diseases. But there is something different about the vaccine for the H1N1 flu, she said.

“;I have looked at the people who are against it, and I find myself taking their side,”; said Wygant Arndt, who lives in Portland, Ore. “;But then again, I go back and forth on this every day. It's an emotional topic.”;

Anti-vaccinators, as they are often referred to by scientists and doctors, have toiled for years on the margins of medicine. But an assemblage of factors around the swine flu vaccine — including confusion about how it was made, widespread speculation about whether it might be more dangerous than the virus itself, and complaints among some health care workers in New York about a requirement that they be vaccinated — is giving the anti-vaccine movement a fresh airing, according to health experts.

“;Nationally right now there is a tremendous amount of attention on this vaccine,”; said Dr. Thomas Farley, the New York City health commissioner. That focus has given vaccine opponents “;an opportunity to speak out publicly and get their message amplified that they didn't have at other times,”; he said.

Barbara Loe Fisher, president of the National Vaccine Information Center, an advocacy group that questions the safety of vaccines, said the swine flu has “;breathed new life”; into the cause. “;People who have never asked questions before about vaccines are looking at this one,”; Fisher said.

The increased interest is frustrating to health officials, who are struggling to persuade an already wary public to line up for shots and prevent the spread of the pandemic. According to a CBS News poll conducted last week, only 46 percent said they were likely to get the vaccine. The nationwide poll, which has a margin of sampling error of plus or minus 3 percentage points, found that while 6 in 10 parents were likely to have their children vaccinated, only 46 percent said they were “;very likely to.”;

“;I wonder if the people disseminating this false information about this vaccine realize that what they are doing could result in some people losing their lives,”; said Dr. Jonathan E. Fielding, the director of the Department of Public Health for Los Angeles County. The comments of vaccine dissenters, which he said “;politically come from the left and the right,”; were frequently, he said, “;not just counterproductive but downright disgraceful.”;

Web sites, Twitter feeds, talk radio and even elevator chatter are awash with skeptics decrying the vaccine, largely with no factual or scientific basis. The most common complaint is that the vaccine has been newly formed and quickly distributed without the benefit of clinical trials; in fact, the swine flu vaccine was made using the same techniques as seasonal flu shots over the last two decades, and a small number of clinical trials were conducted this year to determine the adequate dose.

There are also claims that the vaccine contains adjuvants — sometimes added to make vaccines more effective — although they have not been used in this one. In addition, there is fear that the vaccine could lead to Guillain-Barre syndrome, as was suspected the last time a swine flu vaccine was distributed, in 1976; flu vaccines are now much purer than they were, minimizing the risk, and Guillain-Barre is far rarer.

In measuring the risk of the vaccine, there is general consensus among doctors that serious adverse reactions are very rare and that pregnant women and young people, in particular, are better off with the vaccine than without it. While most people who get H1N1 experience mild symptoms, a recent New England Journal of Medicine study showed that among Americans hospitalized with swine flu last spring, one in four ended up in intensive care and 7 percent of them died.

The illness, unlike other flu strains, has been particularly tough on children and young adults and appears to have a disproportionately high fatality rate in pregnant women. So far, 76 children have died of the virus, according to the Centers for Disease Control and Prevention.

Health care officials are concerned that some groups, especially pregnant women, are potentially swayed by the large-scale efforts of vaccine opponents.

“;One of the things they are focusing on now is immunization and pregnancy,”; said Saad B. Omer, assistant professor of global health at Emory University's Rollins School of Public Health, “;and their perceptions of the vaccine in use of pregnant women. It is not a benign perception in this case, and could have serious impacts, because pregnant women have high risk of complication if they get the swine flu.”;

The anti-vaccine movement, largely comprising activists and a handful of doctors and researchers who connect a variety of health problems — particularly autism spectrum disorders — to vaccines, has failed to find large-scale traction in the United States, where over 90 percent of children are vaccinated.

But at a conference this month of National Vaccine Information Center, Fisher said, there were 675 people, more than double the number at the group's last conference, and half said they were there to discuss swine flu. Unlike most people associated with the center, who have longstanding objections to vaccines or have a connection to the issue because of an autistic child, the newcomers were not traditional skeptics, Fisher said. “;They came not knowing much,”; she said, “;and left galvanized.”;

Further, vitamin vendors — who in some cases operate blogs, with postings by people who claim to be doctors decrying vaccines — are reporting an increase in sales related to swine flu. Michael Angelo, chief research and information officer for vitmens.com who often posts comments on Web sites criticizing vaccines, said sales in September for flu-related products had tripled from last September. The company, he said, has sold 17,565 vitamins said to prevent the H1N1 virus.

Some anti-vaccine groups are also highly organized and quick to respond to openings to promote their message. For instance, this week, an 8-year-old boy from Long Island, N.Y., died roughly a week after receiving a swine flu vaccine, though officials from the New York State Department of Health denied a connection.

Almost instantly, on a memorial page on Newsday's Web site for the boy, Sean Weisse, a message from an anti-vaccine advocacy group appeared: “;We are so sorry to hear about Sean. My understanding, and forgive me if I'm wrong, is that this was a vaccine-related injury. If so, we would like to help you. Best regards, Stan Kurtz Generation Rescue, Jim Carrey and Jenny McCarthy's Organization.”;