"There are no red flags regarding safety," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, noting that a series of clinical trials were being conducted on the vaccine with results due out between mid September and late October.
Adult volunteers participated in the first of five trials starting on August 7, and only reported arm swelling or redness where they received the injection.
Encouraged by the results, health officials went ahead this week with vaccine trials on children aged six months to 17 years, Fauci told reporters.
Additional experiments on about 120 pregnant women -- who are particularly at risk of infection from the influenza A(H1N1) virus and are high on the federal government's priority list for vaccines -- are to begin in September.
Officials expect that between 4,500 and 4,600 people will eventually take part in the vaccine studies.
Between 45 and 52 million doses of the vaccine are expected to be available by mid-October, said Jay Butler, who heads the H1N1 Vaccine Task Force of the US Centers for Disease Control and Prevention (CDC).
The CDC, which had made plans to distribute the vaccine once available, has issued its final recommendations for which groups of people should receive the shot.
The priority groups include "pregnant women, children, and young adults aged 6 months through 24 years, as well as persons aged 25 through 64, who have medical conditions that put them at higher risk for influenza-related complications," he said.
Health care workers, emergency medical service workers and people who take care of infants younger than six months -- too young to be administered a vaccine, should be vaccinated, Butler said.
The novel flu strain, which the CDC says has killed 522 people and caused 7,963 hospitalizations so far in the United States, has generally continued to spread, albeit at a slower speed.
Most of the cases occurred in people under the age of 49.
New infection cases have also decreased in South America and Australia during their seasonal flu season.