As the delta variant of the coronavirus spreads in the United States, some counties are opening community testing sites that they closed last spring when case numbers fell and attention shifted to vaccination.

The demand for tests has increased over the past month. By the end of July, an average of nearly 900,000 coronavirus tests were being performed daily, compared with 500,000 to 600,000 per day at the beginning of the month, according to the U.S. Department of Health.

Several factors are likely to be responsible for the increase, including the spread of the highly contagious Delta variant and new mandates that oblige unvaccinated people to frequent tests. The Centers for Disease Control and Prevention also recently changed their guidelines for vaccinated people, recommending that they be tested if they are exposed to the virus, even if they don’t have symptoms.

Testing has been a hotspot for the United States since the pandemic began. A faulty test, official bureaucracy and delivery bottlenecks initially led to long queues at the test locations and days of waiting for results.

Officials eventually ironed out some of those kinks, and as infections skyrocketed last year, state-run mass testing sites sprang up across the country offering free virus tests to all comers. However, some delays and problems persisted even as capacities increased.

When the vaccines were approved, many large testing centers were converted into vaccination centers and some were closed altogether. Virus testing has largely shifted to the private sector – for example, local pharmacies and commercial laboratories.

“There are far fewer test sites, public test sites, than there were six months ago,” said Mara Aspinall, an expert in biomedical diagnostics at Arizona State University. “So that’s a matter of concern to me.”

After residents began reporting a three-day wait for test appointments at pharmacies in Hillsborough County, Florida, the county opened two free, walk-in test locations last weekend. Officials had planned to run around 500 tests per day at each site and ended up doing almost twice as many, said Kevin Watler, a Florida Department of Health spokesman.

“It’s been very, very busy,” he said. “So the demand is definitely there.”

Many other test sites are emerging in Florida, where the virus is on the rise, as well as across the country. In California, San Diego County added five new test sites last week after traffic increased at its existing sites, officials said.

Other locations are expanding opening hours at testing sites or setting up pop-up testing clinics, and some combine their testing and vaccination services. Last week the Delaware Department of Public Health announced it would begin offering testing at its vaccination sites and is opening a new drive-through testing and vaccination site in New Orleans.

“With the fourth and worst surge in Covid-19 in Louisiana, we need to take a multi-pronged approach to combating the virus,” said Dr. Jennifer Avegno, the director of the New Orleans Department of Health, in a statement. “Masks slow the spread, tests identify cases and pandemic trends, and vaccines prevent hospitalizations and deaths. It only makes sense to put these resources in one place so that residents can access the tools they need for their safety in one place. “