A network of more than 100 clinical trial sites at hospitals and medical clinics in the United States and across the world will take on the unprecedented challenge of testing covid-19 vaccines and other preventive treatments, federal officials announced Wednesday.

The Covid-19 Prevention Network, which knits together the existing federal clinical trial infrastructure developed largely to test HIV vaccines and treatments, launched with a website for volunteers to sign up to be considered when the first trials begin later this month.

The scientific effort to develop a covid-19 vaccine will depend on tens of thousands of volunteers in a gargantuan scientific, medical and logistical undertaking. The aim is to provide “substantial quantities of a safe, effective vaccine by January 2021,” Health and Human Services Secretary Alex Azar said in a statement.

Testing a vaccine is a conceptually simple idea, but it is a careful and methodical process that unfolds through a phased system of trials that grow progressively larger. Early clinical trials, some of which have reported encouraging results, assess the right dose of the vaccine and monitor for any safety concerns in dozens, or a few hundred patients. But the ultimate test of these vaccines will be large trials designed to test whether they are effective at preventing or reducing the severity of the disease.

The first late-stage vaccine trial, in which 30,000 people will be randomly assigned to receive either an experimental vaccine made by the biotechnology company Moderna or a placebo, is expected to begin in the second half of July. There are expected to be at least five such large vaccine trials conducted through the network over the coming months — as well as trials of other preventive measures, such as monoclonal antibody drugs.

“This is what we do for a living and have done for a living,” said Larry Corey, a virologist and past president of the Fred Hutchinson Cancer Center, who is co-leading the trial network. “We have a considerable infrastructure, probably the country’s biggest infrastructure in vaccines.”

Launching trials of this scale in such a short time requires enormous coordination to ensure the data is consistent across many different locations.

In the Moderna trial, 30,000 volunteers will be followed for two years and will be asked to keep a diary of symptoms and be available for weekly check-in phone calls, according to Richard Novak, chief of the Division of Infectious Diseases at the University of Illinois at Chicago College of Medicine. He said he will be recruiting 1,000 volunteers for that study.

“It really is an intense effort,” Novak said. The necessary infrastructure includes equipment that is regularly audited to show that freezers never fail, staff to screen potential participants and a call center to facilitate thousands of check-ins. It also requires building bridges to the community to ensure that diverse groups of people — and particularly those at greatest risk of contracting covid-19 — sign up.

“Getting this up and running is a 24/7 job,” Novak said.