(CNN) -- Days after the U.S. government announced upcoming trials for an H1N1 flu vaccine, Saint Louis University has been inundated with phone calls and e-mails from people volunteering for the study.
Concern about the H1N1 virus grew after it spread quickly around the globe earlier this year.
The university's Center for Vaccine Development has received more than 500 responses from potential volunteers since Wednesday, when the National Institutes of Health (NIH) announced human trials for a swine flu vaccine would begin in early August.
"This response has been exceptionally strong," Nancy Solomon, a spokeswoman for the university's medical center, told CNN Radio. "We haven't had that strong of a response to our call for volunteers since we conducted our small pox vaccine research after September 11."
Thousands of Americans are currently being recruited for swine flu vaccine testing at several research centers across the country, including Saint Louis University's Center for Vaccine Development.
"The federal government comes to us when they need a quick response to test the safety and efficacy of vaccines," Solomon said.
Other trial sites include the University of Maryland Medical Center in Baltimore; Baylor College of Medicine in Houston, Texas; Cincinnati Children's Hospital Medical Center; Emory University in Atlanta, Georgia; Group Health Cooperative in Seattle, Washington; The University of Iowa in Iowa City; and Vanderbilt University in Nashville, Tennessee.
They will be joined by Children's Mercy Hospital in Kansas City, Missouri; Duke University Medical Center in Durham, North Carolina; and IPS Research in Oklahoma City, Oklahoma.
Most of the human trials are being funded by the National Institutes of Health.
In an effort to deliver the vaccine to those who will need it most by October, the clinical trials will enroll as many as 1,000 adults and children, according to officials at the National Institute of Allergy and Infectious Diseases, a division of the NIH, which will lead the effort.
The trials will measure the safety and effectiveness of the vaccine.
The research is a first step toward U.S. health officials' goal of developing a safe and effective vaccine against H1N1, also known as swine flu, which has been declared a global pandemic by the World Health Organization.
After careful screening, volunteers will be inoculated and asked to keep a diary on how they feel. After eight days their blood will be checked. After 21 days they will receive another dose, followed again by diary logs and blood tests.
Patients will be monitored two months for safety issues, followed by a four-month and six-month checkup.
"The purpose of these trials is always to make sure they are safe," said Dr. Karen Kotloff, professor of pediatrics and lead investigator and researcher at Maryland's Center for Vaccine Development. "But even after six weeks, if things look good, we're pretty sure the vaccine will work."
At last count, the CDC said there are 43,771 H1N1 reported cases in the U.S., along with 302 deaths that have been linked to the illness.
In a normal flu season, about 36,000 Americans die from influenza and related complications.
Models predict the 2009 H1N1 flu will peak in October, with many cases being diagnosed in September, according to Dr. Robert Belshe, director of Saint Louis University's Center for Vaccine Development.
"We'll be in the midst of it before we know it," Belshe said.
Swine flu could sicken one in five people this fall, and Belshe said he worries that the number of serious health complications and deaths as a result of the H1N1 virus could soar.
"It's looking more and more like we're going to have a big flu outbreak this fall as soon as the kids get back to school," Belshe told CNN Radio.
"Influenza is unpredictable, but I believe this pandemic will hit pre-teens, teens and their parents hard, and as many as 60 million Americans could be sick with the flu. It's critical that we find a way to protect people from this disease."
CNN Radio's Barbara Hall contributed to this report
All About Swine Flu • World Health Organization • National Institutes of Health