There was cautious optimism today about the rush to get a swine flu vaccine ready for the fall flu season.
"We are on track in development," said Dr. Marie-Paule Kieny, director of the World Health Organization's initiative for vaccine research.
But questions persisted about whether there will be enough doses to go around and whether they'll be safe.
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Kieny stopped short of predicting how much vaccine would be available this fall, citing setbacks that have come from raw material from seed strains yielding less than expected.
"In terms of the capacity, I would really like to avoid making any projections right now," Kieny said.
"Indeed, we have had news and discussions with all manufacturers, and they were reporting yields that were between one-third and one-half of what they usually get for a good seasonal strains," she said.
"The latest results that we have heard as of this week is that at least one of the strains that has been produced seems to be promising and seems to give equivalent or similar yields as the ones that the manufacturers have for seasonal vaccines," she said. "It really seems that we have found way to go around the problem."
Before facing that issue, Kieny said WHO had predicted that in the best-case scenario, manufacturers would have been able to churn out 94 million doses of the vaccine per week once production started. If the yield were cut in half, production of doses would be cut in half, too, she said.
"We prefer to stay on this best-case scenario and to give a much better and really a firm response when the yields are known," Kieny added.
Some Clinical Trials Results Could Come in September
Clinical trials already under way in some countries will help confirm, among other things, how many doses might be needed per person.
In some European countries, manufacturers will be able to submit safety profiles that would attest to the quality of similar vaccines in lieu of time-consuming clinical trials. The United States is required to first test vaccines in clinical trials, and could begin those trials within the week. Results from those tests would be expected at the end of September.
It could take another four to six weeks to begin a vaccination program after clinical trials' results come in -- meaning that in the United States, the start of a vaccine program could come in November. Guidelines for U.S. school closures are also expected to come this week from the federal government. The U.S. government wants nearly 160 million Americans to get a swine flu vaccine.
China, Australia, the United Kingdom and Germany are also among the first out of the gate to begin clinical trials. Kieny said those that started in July should provide early results in the first half of next month.
The tests will confirm whether people would need one or two shots of the vaccine for immunity, and will examine whether there are any dangerous side effects related to getting immunized.
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Safety Concerns Persist About Safety of Swine Flu Vaccine
Pelted by questions about possible side effects of anticipated vaccines, Kieny tried to temper concerns that an aggressive timeline for vaccine production could compromise people's safety.
"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.
Developing Countries Expected to Receive Adequate Swine Flu Vaccine
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.