Health experts are signaling increased urgency over the need to build an army of people to trace the spread of coronavirus, as states try to put together a patchwork network in an effort to contain the disease.

Centers for Disease Control and Prevention (CDC) Director Robert Redfield warned Thursday that the country needs between 30,000 and 100,000 people to do contact tracing by September to try to stop a major resurgence of the virus in the fall and winter.

Other estimates have put the need even higher, with former CDC Director Tom Frieden calling for up to 300,000 contact tracers.

There are varying levels of success so far as states and localities take the lead in putting together the workforce, with some places still lacking the manpower to conduct tracing.

“There are communities that don’t have the resources they need to do this work,” said Lori Freeman, CEO of the National Association of County and City Health Officials. “This is the critical component of getting our economy back.”

In combination with widespread testing, contact tracers are used to interview people infected with coronavirus to find out who they have been in close contact with, then reach out to those people and encourage them to quarantine for 14 days to prevent further spread of the disease.

“While testing has gotten a lot of attention and rightly so, and while our country continues to have an insufficient number of tests, what has received less attention has been contact tracing and the importance of contact tracing,” Ashish Jha, the director of the Harvard Global Health Institute, said at a congressional briefing on Thursday.

Congress provided $25 billion for testing in a response bill at the end of April, and that money can also be used for contact tracing. But the Association of State and Territorial Health Officials is calling for $7.6 billion in new federal funding dedicated solely to contact tracing.

Bureaucratic hurdles are also posing a problem in some cases for scaling up contact tracing.

For example, Kansas City, Mo., does not have enough contact tracers and has had trouble accessing funding under the CARES Act passed by Congress because it sits in four different counties and is needing to negotiate for funds with each county, according to Morgan Said, a spokeswoman for Mayor Quinton Lucas (D).

In California, officials are short of the initial goal of 10,000 contact tracers and the ultimate goal of 20,000 set by Gov. Gavin Newsom (D). The Department of Public Health said Friday that the state currently has about 3,000 staff within local health departments and is in the process of training another 3,400 to be contact tracers.

The state is off to a “slow start” in its effort to train current workers to be redeployed as contact tracers, Abby Snay, deputy secretary at the California Labor and Workforce Development Agency, told lawmakers at a briefing on Thursday.

Illinois Gov. JB Pritzker (D) said at the end of May that only about 30 percent of people who should be contacted due to their interaction with infected people are being reached, but said that the state would be working throughout June to hire more people. “It’s a large endeavor,” he said.

Texas also fell short of the goal of 4,000 contact tracers that Gov. Greg Abbott (R) set for June 1, and currently has roughly 2,900, according to the Texas Tribune.

Other states have reported more success.

Massachusetts was an early leader in ramping up contact tracing and has partnered with the humanitarian organization Partners in Health for over 1,500 contact tracers. There are an additional 1,000 contact tracers at local health departments, the state said.

Maryland said it had quadrupled its contact tracing workforce to about 1,400 since late April.

While new cases of coronavirus nationally have fallen some from their peak, there are still about 20,000 new cases reported a day, with surges in some states like Arizona, Texas and Arkansas.

Redfield, the CDC director, pointed to AmeriCorps as another source of contact tracers at the hearing on Thursday.

AmeriCorps has partnered with Colorado for over 800 contact tracers, the state announced this week. The organization said it is working to announce agreements with a handful of other states as well.

Redfield expressed urgency for getting enough contact tracers in place ahead of a potential second wave in the fall.

“We really have to get this built and we have to get it built between now and September,” he told the House Appropriations Committee.

One obstacle is that if there are too many new cases of coronavirus each day, the contact tracing system becomes overwhelmed.

Experts say the system works best in trying to keep a relatively small number of cases from spiraling out of control. Therefore, if the number of cases remains too high throughout the summer, it hinders efforts to tamp down a resurgence in the fall.

Still, Redfield expressed hope that the country could contain the virus in the fall.

“It is fundamental that we have a fully operational contact tracing workforce that can every single case, every single cluster, do comprehensive contact tracing within 24 to 36 hours, 48 hours at the latest, get it completed, get it isolated, so that we can stay in containment mode as we get into the fall and winter of 2020,” he said.