Connecticut had at least six times the number of people infected with COVID-19 as the state reported — an estimated 167,700 residents infected by the disease at the beginning of May, the  Centers for Disease Control and Prevention said Friday.

The CDC said its first antibody study of 1,431 Connecticut residents indicated that nearly 5% percent of the state’s population had antibodies to COVID- 19 at the beginning of May. Connecticut was the second-highest of five states tested for antibodies in the agency’s first study. Only New York showed a higher rate of infection, nearly 7%.

The blood samples in Connecticut were taken from April 26 to May 3. On May 3, the state reported that 29,300 residents had tested positive for the coronavirus, about one sixth of the infections the antibody tests discovered.

The CDC partnered with commercial laboratories to conduct a large-scale geographic seroprevalence survey that tested for COVID-19 antibodies in clinical blood specimens from Connecticut, South Florida, the New York City metro area, Missouri, Utah and Western Washington State.

The survey includes people who had blood specimens tested for reasons unrelated to COVID-19, such as for a routine or sick-visit blood test by commercial laboratories. The. CDC aims to test about 1,800 samples collected from each testing area about every three to four weeks to determine what percentage of people tested already have antibodies against COVID-19, and how that percentage changes over time in each area.

The CDC plans to expand the survey to an additional four states, including California, Louisiana, Minnesota, Pennsylvania and Utah.

Increased use of antibody testing will provide a snapshot of how many Americans actually had the disease even when they did not think they ever had it, experienced few or mild symptoms or no symptoms at all.

On Thursday, CDC Chief Robert Redfield said that nationally coronavirus infections are likely 10 times higher than the 2.4 million determined by testing.

While it’s not 100 percent precise, antibody testing is thought to provide the fullest snapshot of the prevalence of COVID-19  in any given place.

Their suspicions confirmed

Jennifer Vandergraaf and her husband are among those in Connecticut who had COVID-19, but were never in the state’s daily count of positive cases.

Near the end of March, the couple did not feel well. Vandergraaf, 56, developed a cough and tightness in the chest. Her husband had deep body aches and a stomach upset. But the Vandergraafs, who live in New Canaan, initially did not think they had contracted COVID-19 because they did not have a high fever or other signs that were associated with infections.  So they did not get tested, especially since the tests were scarce at the time.

But because of lingering doubts – especially since they both lost their sense of taste and smell — Vandergraff and her husband eventually underwent antibody testing in May. Both tests came back positive for COVID-19.

Now the couple is recovered and Vandergraaf says she’s glad they underwent the testing.

“I have family with serious medical issues and I wanted to get tested,” Jennifer Vandergraaf said. Since the test determined they have coronavirus antibodies, she hopes she and her husband have some immunity to the disease, at least in the short term.

That’s perhaps the biggest question antibody testing might be able to answer – whether someone who has been infected with COVID-19 has developed any immunity to the disease.  There’s no definitive answer yet.

“We are still learning,” said Dr. Ajai Kumar, chief clinical officer at Hartford HealthCare.”We haven’t spent a lot of time with COVID-19 to know how long any immunity will last.”

Meanwhile Rick Torres, Yale University assistant professor of laboratory medicine, said “it is still very difficult to say much about it means to have had about 5% of Connecticut residents with detectable antibodies in May.”

”

We all really want to know how many people have been exposed to the virus, who is immune or still vulnerable to infection, and how long will immunity last,” Torres said. “Unfortunately, the antibody results, even if collected in a statistically valid way, do not give a lot of information about any of these. ”

The Connecticut Department of Public Health said that, based on electronic laboratory reporting, 23,115 state residents like the Vandergraaf’s had  COVID-19 serology testing done in May.

Blood can be drawn for an antibody test in a doctor’s office, hospital or laboratory. Jennifer Vandergraaf had her test at her doctor’s office. Her husband took advantage of testing offered by the town of New Canaan.

Testing locations can be found on ct.gov/coronavirus. Kumar said Hartford HealthCare is in a partnership with Quest Diagnostics to provide antibody testing to all with a doctor’s order.

“Some individuals are just curious, others have had a flu-like illness and want to know if they’ve been infected,” Kumar said.

Although some antibody tests have been criticized for generating false positives and false negatives, Kumar said Hartford HealthCare’s tests are “sensitive” and accurate. “They give good results,” he said.

But Torres said that those who are exposed to virus vary quite a bit on the amount of antibody they produce.

“It is becoming more and more apparent that those with few or no symptoms who have virus detected by the direct virus test produce less antibody, sometimes not producing enough to be detectable at all,” he said

He also said antibody tests cannot determine the “type” of antibody a body produces after it comes in contact with the coronavirus.

“By type of antibody I mean the part of the virus to which the antibodies are targeting,” Torres said. “And the different tests for antibodies out there vary in which of the antibodies they detect. So that means that antibody tests likely cannot detect every exposure and not every antibody test detects at the same rate, even if the test is technically perfect.”

Besides offering tests to the public, Hartford HealthCare is testing all of its employees. Kumar said about 3,000 tests have already been conducted.

And Hartford HealthCare is among several medical facilities conducting clinical trials to determine whether laboratory-designed antibodies are effective at preventing and treating infections from the novel coronavirus.