Just a third of the vaccine needed is likely to be ready by then as government health officials mobilize to vaccinate at least half of the country's population, The Washington Post reported Sunday.
"This is potentially the largest mass vaccination program in human history," said Howard Markel, a medical history expert at the University of Michigan who is advising the U.S. Centers for Disease Control and Prevention.
The lag in vaccine production and the rush to test it for safety and effectiveness shows the potential for complications in trying to immunize so many people so quickly, health officials told the Post.
Not yet known is how many shots people will need, what the proper dosage should be and how to avoid confusing people with an overlapping effort to fight the regular seasonal flu, the Post reported.
A cohesive operation is crucial to defending against the second wave of the influenza pandemic that began in Mexico last spring, health officials said.