FRIDAY, Sept. 4, 2009 (HealthDay News) -- Despite persistent public concern that H1N1 vaccines being tested might have unforeseen side effects, experts reiterated Thursday that so far, at least, the shots seem safe.
"There have been no red flags at all," Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, told HealthDay.
A trial designed to test the vaccine on healthy adults began on Aug. 7, while another trial involving children started Aug. 18.
"We wanted to wait about a week after the first trial to see if there were any glaring safety issues. There were not, so one-and-a-half weeks later, we started the trial in children," Fauci said.
And Fauci is optimistic that the vaccine will continue to be safe.
"It's unlikely there will be safety issues," he said. "We don't expect any because this vaccine is strikingly similar to the seasonal vaccine we give every year to millions and millions and millions of people."
Still, rare side effects will only come to light after millions of people have been vaccinated and after different population subgroups have been exposed to the vaccine.
"No clinical trial in the world is going to be large enough to be able to detect an event that occurs one in every 100,000 people," Fauci said. "The only way to pick that up is after the fact, when a lot of people get vaccinated and good surveillance picks it up."
Another expert agreed. "One of the challenges is that sometimes on the safety issue, only large-scale use will uncover rare events," said Dr. Melinda Moore, senior health researcher with the RAND Corporation in Arlington, Va., and a former epidemiologist with the U.S. Centers for Disease Control and Prevention. "That's not unique to this product. That's why there's so much after-market surveillance."
Such a surveillance system will be in place and operable when the first vaccinations start to roll out. Fauci anticipates that 45 million to 50 million doses will be available by Oct. 15.
Many people are painfully aware of a U.S. government decision in 1976 to vaccinate 43 million people against swine flu -- it backfired badly.
Not only did the dreaded outbreak never materialize, but some 500 Americans who did get vaccinated came down with a rare neurodegenerative condition called Guillain-Barre syndrome, which many experts believe was linked to the shot. Twenty-five of those 500 people died.
One thing that's already different about this year's H1N1 virus: The 1976 virus never spread beyond 240 soldiers stationed at Fort Dix, N.J., while the current outbreak has already sickened more than 200,000 people worldwide, killing 2,200 or more, according to the World Health Organization.
"That  virus never evolved," Moore said. "People thought it was going to be a new pandemic. It didn't turn out to be. The threat wasn't as big as they thought. This virus has already demonstrated that it has spread."
Still, segments of the public may be wary of any vaccine they perceive as being rushed out too fast. A report in a recent issue of the BMJ found that more than half of all health-care workers surveyed in Hong Kong have already decided not to get vaccinated because of fears about safety and doubts on efficacy.
U.S. officials do not yet know whether the vaccine will require one or two doses. "We will know in a week or so if one dose is going to be enough," Fauci said. "We're hoping it's going to be one."
Reports published Thursday by drug maker Novartis suggest its vaccine may be effective after just one jab.
If two doses are needed, researchers will need to figure out how far apart they should be given, Moore said, and whether an adjuvant -- something to boost the immune response -- is called for.
"They will have to balance it all out," she said. "Ultimately the product needs to be licensed. This is not a theoretical exercise. The virus is here and so is the U.S. population -- and so is the flu season."
Find out more about the H1N1 swine flu at the U.S. Centers for Disease Control and Prevention.