"This whole process is moving ahead very briskly, but very, very rigorously," said Dr. William Schaffner, chairman of the Department of Preventive Medicine at Vanderbilt Medical School in Nashville, Tenn. "There's not a corner being cut."
Kieny said that while "there is no doubt" some people will experience side effects, many of them will not be directly related to receiving the vaccine. She said each country's regulatory health agency will be keeping a close eye on whether adverse reactions are "linked to the vaccine or whether they are just coincidences."
Kieny added that data from past trials on similar vaccines will also provide support for the safety of the new vaccine.
"Part of the vaccines are based on very old and proven technology which are used for seasonal vaccinations," Kieny said.
While the new vaccine will likely have the same known side effects as other vaccines, such as arm pain, fever, nausea and other complaints, more serious side effects are possible, though unlikely.
In particular, health officials will keep close watch on the slim possibility of Guillain-Barre syndrome, the rare neurological condition that accompanied immunization against the swine flu of 1976 in the United States. The rare side effect killed dozens of people out of the estimated 40 million who received the vaccination.
Kieny called today's vaccine "much purer" than the one used in 1976.
She also said there's no reason to be worried for the time being about receiving a seasonal flu shot and swine flu shots in quick succession or giving a swine flu vaccine to people who have already caught the virus.
She added that the use of adjuvants -- substances added to vaccines in order to boost immune response and allow the dose supply to be stretched to cover more people -- would also be a safe approach. There is not an adjuvant flu vaccine currently registered in the U.S.
Still, the U.S. government has said any rare complications may not show up until after the fact.
"There's only so much information we can learn about safety before vaccines go into people, because they're not tested in millions of people," Dr. Anne Schuchat of the Centers for Disease Control and Prevention said last month.
Kieny also said WHO is negotiating with manufacturers to ensure that even the poorest countries have access to enough of the vaccine to cover vulnerable populations.
"We will assure a minimum level of coverage in all low-income countries," she said.
ABC News' Lisa Stark contributed to this report.