Thursday, May 26, 2022

What’s the difference between Covid-19 and antibody tests, and can the latter prove immunity? [Free read]

WILMINGTON — Simply put, Covid-19 diagnostic tests are designed to detect current infections of the novel coronavirus, while antibody tests are designed to detect past infections.

Although scientists and medical experts are still trying to figure out whether people already exposed to the novel coronavirus are immune from a second infection, antibody tests are on the rise throughout the U.S. as numerous states consider re-opening their economies.

Effective antibody tests could potentially show whether people are no longer at risk of contracting the virus themselves or passing it along to others, which can be done with or without symptoms present. But the key word here is ‘potentially.’

RELATED: New Hanover County to let additional restrictions lapse on Wednesday, ramps up testing

Many experts around the world are skeptical as to whether current antibody tests on the market are reliable and whether they actually prove immunity until an effective vaccine is discovered and globally distributed.

Last Friday, the World Health Organization published a scientific brief warning against governments who are now suggesting that the detection of antibodies to the virus “could serve as the basis for an ‘immunity passport’ or ‘risk-free certificate’ that would enable individuals to travel or to return to work assuming that they are protected against re-infection.”

“There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection,” according to the report.

Dr. Paul Kamitsuka, head epidemiologist at New Hanover Regional Medical Center, said the major problem with current antibody tests is a “lack of specificity.”

“[T]hat is, some of the tests may cross-react with other coronaviruses that cause the common cold, and therefore may be false-positive,” he said in an email sent Monday afternoon. “We are hoping that better, more specific, tests will be forthcoming.”

But he did suggest that current antibody tests are showing some level of immunity.

“The current thinking is that if you are seropositive (positive antibodies), then you may be immune to re-infection at least for some period of time,” Dr. Kamitsuka said. “We do not yet know for sure if this is the case,  however, and if it is, for how long.”

In the meantime, antibody tests play a crucial role in helping map out the spread of the virus by showing who may have already contracted it — with or without symptoms — while also helping understand the extent of infection.

The U.S. coronavirus death toll surpassed 50,000 last Friday, according to numbers compiled by Johns Hopkins University. As of Monday afternoon, there were 55,637 deaths reported and just over 980,000 confirmed cases in the country.

Below, find answers to some of the biggest questions people are now asking about the difference between the two types of tests and the issues with each.

What is the difference between an antibody test and a Covid-19 test?

Dr. Paul Kamitsuka, head epidemiologist at New Hanover Regional Medical Center, helped explain the difference between antibody tests and Covid-19 tests. (Port City Daily photo/Ben Brown)

Antibody tests reveal if people’s blood contains antibodies — a protein the body produces in response to an antigen like the novel coronavirus — while Covid-19 tests show whether a person is actively infected with the virus at the time the test was conducted.

An antibody test, known as a serology test, can determine whether a person had been infected and has recovered — when the virus is no longer present in the body. In other words, it detects the presence of the human body’s immune response. Antibodies provide immunity to fight the virus, lowering the risk of getting sick.

A polymerase chain reaction (PCR) test, on the other hand, detects the presence of an antigen like the novel coronavirus, which causes the disease known as Covid-19. A PCR test can also detect whether a person has the virus even before symptoms of the disease are present.

PCR tests, which typically use nose or throat swabs, are crucial in determining who is currently infected and warn others they have had contact with to quarantine, helping break the transmission chain of the highly contagious virus.

“Genetic material (RNA in this case) is detectable starting 1-2 days prior to the onset of symptoms in most patients,” Dr. Kamitsuka said. “It does not directly assess whether the patient is shedding viable contagious virus, although that is assumed in most cases.”

Do antibody tests prove immunity?

This is the biggest concern, and until scientists and physicians have a larger data pool to better understand the new virus, the effectiveness of antibody tests is a moot point until any evidence is discovered that people can develop the necessary antibodies to fight it off themselves and prevent it from spreading.

According to the WHO, the development of immunity to a pathogen like SARS-CoV-2 (the official name of the novel coronavirus) typically lasts one to two weeks.

“WHO continues to review the evidence on antibody responses to SARS-CoV-2 infection. Most of these studies show that people who have recovered from infection have antibodies to the virus. However, some of these people have very low levels of neutralizing antibodies in their blood, suggesting that cellular immunity may also be critical for their recovery.”

The WHO report said that as of April 24, no study has yet evaluated “whether the presence of antibodies to [the novel coronavirus] confers immunity to subsequent infection by this virus to humans.”

Furthermore, data from South Korea and China — two of the earliest known hot spots — “have shown that a growing number of recovered patients appear to have suffered a relapse of the disease,” according to reporting by NPR.

“It remains unclear why this is occurring — whether it is a sign of a second infection, a reactivation of the remaining virus in the body, or the result of an inaccurate antibody test,” NPR reported.

Are antibody tests reliable?

The same WHO report also said that lab tests designed to detect antibodies, including rapid immunodiagnostic tests, need “further validation to determine accuracy and reliability.”

“Inaccurate immunodiagnostic tests may falsely categorize people in two ways,” according to the report. “The first is that they may falsely label people who have been infected as negative, and the second is that people who have not been infected are falsely labeled as positive. Both errors have serious consequences and will affect control efforts.”

Because people could be infected with any one of six human coronaviruses that are known to exist today, including SARS-CoV-2 and four others that cause the common cold, the WHO said that antibody tests “need to accurately distinguish between past infections from [the novel coronavirus]” and infections of the other viruses.

The report warned that those infected by any one of the six coronaviruses may develop antibodies that cross-react with antibodies produced to counteract the novel coronavirus, just as Dr. Kamitsuka said.

The WHO said it supports studies conducted by many countries to test antibodies to the novel coronavirus, “critical for understanding the extent of – and risk factors associated with – infection.”

But while these studies will provide useful data on population percentages with detectable antibodies, “most are not designed to determine whether those people are immune to secondary infections” of the novel coronavirus.

On the West Coast, a University of California, Berkeley and UC San Francisco project evaluated a dozen of more than 120 available antibody test kits now available.

Although researchers discovered many of the tests “performed reasonably well,” many of the test kits “have false positive rates that may exceed the proportion of people who have been infected in some communities.” In other words, many who test positive on these antibody tests “may not actually have had Covid-19.”

“It’s the Wild West right now,” UC Berkeley assistant professor Patrick Hsu said. “These tests are widely available, and many people are buying and deploying them, but I realized that they had not been systematically validated, and we needed to figure out which ones would really work. This is a huge, unmet need for public health.”

According to, tests are not yet widely available while many that are available are found to be ineffective.

“The U.K., for instance, spent $20 million on antibody tests from China that the government subsequently found were not accurate enough to use. An emergency room in Laredo, Texas, also dropped half a million dollars on tests,” the website reported.

Are antibody tests useful in mapping spread of the virus?

According to Dr. Kamitsuka, the answer is yes:

Antibody tests would be most useful in doing surveys of populations, to see what percent of the community appears to have already had COVID-19, whether clinically recognized or not,” Dr. Kamitsuka said. “In New York, for example, early data indicates that almost 14% of persons tested in NY state have positive results, and about 20% in NYC.

We would like to do similar studies in North Carolina, but await better tests to avoid the cross-reactivity problem. A key implication of serology surveys showing only a fraction of the population being seropositive is that it means that most people are still susceptible to getting COVID-19 infection if exposed to the virus.

Where can I get Covid-19 tests and antibody tests?

There are laboratory tests that can identify the virus in respiratory specimens. According to a study led by the Yale University School of Public Health, saliva samples are preferable to deep nasal swabs, although Dr. Kamitsuka said the latter are more commonly used.

State and local public health departments have received tests from the Centers for Disease Control and Prevention (CDC), while medical providers are receiving tests developed by commercial manufacturers, according to the CDC.

According to USA Today, The U.S. Food and Drug Administration (FDA) has authorized at least four antibody tests, including one developed by Cellex, a medical diagnostics company headquartered in the Research Triangle Park in the Raleigh area. The tests were given ’emergency use’ authorization, which allows unapproved medical products to be rolled out during a public health crisis.

According to Cellex, their tests are only available to health care workers and labs, not to private individuals or home testing.

The New York Times on Sunday reported that the National Institutes of Health, the CDC, “and virtually every university with an epidemiology department has begun antibody surveys in communities across the United States.”

What’s happening in New Hanover County?

On Sunday, New Hanover County said it aims to ramp up Covid-19 testing so that approximately 1% of the county’s population — or around 2,400 people — will be tested in the next four weeks.

Lisa Brown, Public Health Preparedness Coordinator for New Hanover County, said she wasn’t aware of “this level of robust testing and commitment to the understanding of Covid-19 in any other county in the state.”

Private physicians in the region along with New Hanover County Medical Center are also performing tests.

Additionally, Wilmington Mayor Bill Saffo announced a large, anonymous donation of antibody testing kits and N95 face masks. He said plans for the donated tests are not yet laid out, but the city and county will be working together to accomplish that.

Send tips and comments to the reporter at, @markdarrough on Twitter, or (970) 413-3815

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