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.
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.
Visit the ABC News OnCall+ Swine Flu Center to get all your questions answered.
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.