Houston reported 615 new coronavirus cases Thursday — a 62 percent daily increase and the largest single-day spike of the pandemic — as well as two additional deaths as health officials remain hampered by limited testing and delayed results.

Harris County reported 91 new cases and one new death, an African-American woman in her 80s. Countywide, there have been 3,047 confirmed cases and 34 deaths as of Thursday evening. Almost 420 ill residents have recovered.

Mayor Sylvester Turner said half of the new Houston cases came from March tests, with the remaining from the past nine days. The city received a “large volume” of reports electronically from testing labs and hospitals. Backlogs have led to other single-day bursts, as well, including 402 new cases reported on Monday.

The unpredictable lag between a test and confirmed case complicates efforts by epidemiologists to determine whether the virus is reaching its peak here. Houston-area hospitals are bracing for a surge in cases and Harris County by Friday plans to complete a makeshift field hospital next to the Astrodome for use as a last resort.

Turner also announced the two new deaths, for a total of 14 involving Houston residents. One was a white man in his 80s who died March 27. The other was a Hispanic woman in her 70s who died April 2. Like all who have died with the virus so far in Houston, they had underlying health conditions.

Now Playing:

The situation at the Harris County Jail, meanwhile, grew more worrisome for Sheriff Ed Gonzalez, who reported a fifth inmate had tested positive for the disease. The number of stricken Sheriff’s Office employees rose to 40, three-quarters of whom work at the jail.

A plan by County Judge Lina Hidalgo to release some nonviolent arrestees, in an effort to slow the outbreak there, remains in limbo six days after a criminal court judge voided her order.

The number of Houston’s confirmed cases had paled in comparison to other large cities, a fact Turner has emphasized while at the same time cautioning that testing has not been even close to fully revealing the virus’ spread here. Turner has, on several occasions, advised Houston residents to multiply the number of known cases by 10, due to the lack of robust testing.

Houston received good news on that front Thursday. The federal government said it would support the four testing sites here through the end of May, amid reports that other sites in the country have been shut down.

Dr. David Persse, the city’s health authority, said the lag in numbers was due to incoming data from testing labs, which he said has led to a “sawtooth pattern” in the city’s rise of cases.

“It’s frustrating for all of us to see these numbers behave this way. It’s a function of the delay in the reporting,” Persse said. “There’s not necessarily a bad actor here. Some labs are just taking longer to get the results. And sometimes, they do cohort it before they report it to us. The law in Texas is that they’re supposed to report the positives to us immediately.”

He said there is sometimes a delay on the city’s side, too, as health workers confirm that each case involves someone who actually lives in Houston and parse out those that do not. That logistical hang-up, he said, would not delay reporting by more than a day.

He challenged every Houston resident to be more stringent in social distancing.

“Quite honestly, what the jump today tells us is we need to do a better job with what we’re doing,” Persse said. “As individuals, we need to do a better job.”

The large jump led Turner to reverse course and close city parks, a move he had resisted just one day earlier, saying he did not want to be heavy handed. The county on Wednesday announced it would close its parks for the Easter weekend, worried about crowds gathering in defiance of the stay-at-home order.

Houston Police Chief Art Acevedo asked the community for continued prayers for Ramon “Ray” Cervantes, a 57-year-old homicide detective who remains in intensive care on a ventilator with COVID-19, the disease caused by the new coronavirus. Cervantes was in “really grave condition” but has seen some slight improvements, according to the chief. His wife, Liz, also has tested positive and is isolating at home, Acevedo said.

Harris County for the first time reported a breakdown by race and ethnicity of its 20 deaths. The fatalities, which exclude the city of Houston, include: Nine white, four Hispanic, four black and one Asian. The remaining two are categorized as “other,” which includes more than race, Pacific Islander and Native American.

Combined with the Houston deaths, the racial and ethnic breakdown of all coronavirus fatalities in Harris County is:

35 percent black

35 percent white

20 percent Hispanic

3 percent Asian

6 percent other

Blacks, who represent just 19 percent of county residents, are overrepresented. Whites, who account for 29 percent of the population, are as well, though to a lesser degree.

dylan.mcguinness@chron.com

zach.despart@chron.com