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Why you shouldn’t get a COVID antibody test after a vaccine

Many Americans are basking in the warm light of what appears to be the end of the coronavirus pandemic. Millions of people are vaccinated, and restrictions that had prevented further spread of COVID-19 are loosening. People are planning summer travel and long-delayed reunions with loved ones. 

That optimism, however, is tinged with uncertainty — about the effectiveness of vaccines, the possibility of getting or transmitting the virus even after vaccination, and the rise of variants that may spread more easily or evade vaccines altogether. Some people are thinking about antibody tests as a way to check how effective their vaccines are.

READ MORE: What COVID vaccine side effects can and can’t tell you about your body’s immune response

But antibody tests don’t paint a full picture. They are not an effective way of making sure your vaccine and immune system are on task, said Dr. Jesse Goodman, who formerly directed the Food and Drug Administration’s Center for Biologics Evaluation and Research and is a senior scholar with the O’Neill Institute for National and Global Health Law. 

“I definitely would not recommend it,” he said. “For most people, if you’ve been vaccinated with an authorized vaccine, you’re going to have antibodies, and we understand what your protection would be.”

These tests, experts say, have their place. But they caution against using them as a tool to determine your individual level of risk.

What are antibody tests?

There are different ways to detect the presence of coronavirus; timing dictates which approach is best. Diagnostic tests for COVID-19, such as polymerase chain reaction, or PCR,  detect viral genetic material present in the mucus and saliva. In contrast, antibody tests — or serological tests — are designed to detect whether or not a person had been previously infected with the virus and has since recovered.

These antibodies usually show up seven to 11 days after infection. According to the FDA, it remains unclear how long antibodies stick around in someone’s body after getting infected with the virus (studies suggest some antibodies could persist for up to six or eight months), or how much protective immunity those antibodies offer.

Antibody tests are performed by drawing a blood sample from someone, which is analyzed in a laboratory to see if there are antibodies present, a sign the person’s immune system recently fought off the virus. A negative result means there were not enough antibodies in a person’s blood to signal an earlier COVID-19 infection. These tests are especially useful in determining if a person may have been exposed to the coronavirus but developed no discernible symptoms.

Like COVID tests for current infection, serological tests are covered by private and federal insurance plans.
The FDA says these tests should only be ordered by clinicians who know how to interpret their results and also are aware of the limitations. 

What an antibody test can and can’t tell you

Antibody tests are not one-size-fits-all, which raises another consideration prior to taking such a test. When someone is infected with the coronavirus, their immune system produces antibodies against the nucleocapsid, where the virus’s genetic material is encased, and spike proteins that exist around the outside of the virus. 

According to studies cited by the Centers for Disease Control and Prevention, those resulting antibodies may offer some protection for roughly three months, but it is unclear how long that protection lasts. 

The FDA has authorized 42 antibody tests for emergency use. Some of these tests have been criticized for being imprecise tools that are not as sensitive or specific as they should be to produce results that can then be interpreted in a meaningful way. 

Since these tests were authorized for emergency use during the pandemic, Goodman said, “We don’t have a tremendous amount of experience with them to know how they correlate with protection.” 

Even in the earlier stages of the pandemic, when researchers looked to antibody testing as a critical tool to understand the spread of COVID-19, there were doubts about their reliability. At the time, some people were personally interested in getting tested to find out whether they’d already contracted the virus without showing symptoms and might be immune to it — or whether what they thought was a mild flu in February 2020 was actually COVID. 

Now, some are curious to explore the tests as a way of checking whether their vaccines are working.

Public health officials are urging fully vaccinated people not to place weight on these tests — or even get them in the first place. On May 19, the Food and Drug Administration cautioned against the use of antibody tests to check whether or not one’s COVID-19 vaccine had generated a sufficient immune response to protect them against infection. In a written statement, Dr. Tim Stenzel with the FDA’s Center for Devices and Radiological Health said these tests have “limitations.” 

“Antibody tests can play an important role in identifying individuals who may have been exposed to the SARS-CoV-2 virus and may have developed an adaptive immune response,” he said. “However, antibody tests should not be used at this time to determine immunity or protection against COVID-19 at any time, and especially after a person has received a COVID-19 vaccination.”

The FDA warns that misinterpreting antibody test results could lead people to take fewer precautions than necessary, or cause needless worry. The agency points out that the vaccines themselves can lead to positive antibody results on some tests. 

Conversely, a lack of antibodies detected does not mean that a vaccinated person did not have a protective immune response. The COVID-19 vaccine trains immune cells to notice and attack the signature spike protein. Antibody tests may look for different antibodies that are only produced after natural infection. “Therefore, COVID-19 vaccinated people who have not had previous natural infection will receive a negative antibody test result if the antibody test does not detect the antibodies induced by the COVID-19 vaccine,” according to the FDA.

The three COVID-19 vaccines authorized for use in the United States are very safe and effective, according to mounting evidence, both from robust clinical trials and real-world data. The two-dose vaccines from Pfizer and Moderna, which rely on mRNA technology, are more than 90 percent effective at preventing infection from the coronavirus. And the Johnson & Johnson vaccine, which uses conventional adenovirus vector technology, is more than 70 percent effective in a single shot.

Nearly 60 percent of Americans age 12 or older have received at least one vaccine dose, according to the latest data from the CDC, and 40 percent of the nation’s total population is fully vaccinated. From early January to late May, the country has gone from reporting around 250,000 new daily infections to recording under 20,000 new cases per day. 

Questions of vaccine effectiveness in people with compromised immune systems

Vaccinations have played a critical role in bringing down those rates of new infection, hospitalization and death, easing pressure on vaccinated individuals to adhere to strict social distancing and behavioral measures. On May 13, Dr. Rochelle Walensky, who directs the CDC, announced that those who are fully vaccinated “can shed your mask.”

But an often-overlooked part of that new guidance cautions that people who have compromised immune systems — either because of a condition that affects the immune system directly, or because of treatment that works by suppressing the immune system — may still be at risk even after vaccination. For people whose immune systems don’t function normally, the vaccination process of training immune cells might not always play out successfully.

“If you have a condition or are taking medications that weaken your immune system, you may NOT be fully protected even if you are fully vaccinated,” the new guidance says. “Talk to your healthcare provider. Even after vaccination, you may need to continue taking all precautions,” including wearing masks.

That uncertainty has led some immunocompromised people to take antibody tests to check if their immune system is responding to the vaccines. 

“I am hearing that immunocompromised individuals are doing that and some physicians are doing that for their patients,” Goodman said.

The coronavirus pandemic is not the first time questions about vaccines work in particular populations. The influenza vaccine prompts similar concerns, but immunization is still standard care. 

For millions of Americans who live with compromised immune systems, the science is still catching up to determine exactly how effective COVID-19 vaccines are for them, Goodman said. 

A recent analysis published in JAMA that said nearly 3 percent of Americans take medications that weaken their body’s immune response — basically telling their body to back down and not fight. These medications include chemotherapy, steroids — such as prednisone, used to treat arthritis, multiple sclerosis and more — and immunosuppressants given to organ transplant patients.  

READ MORE: ‘It feels like freedom,’ 8 people describe getting their COVID-19 vaccine

“What immunosuppressive drugs do — the reason that they work — is because they reduce the ability of your immune system to recognize and fight off things that it perceives to be threats,” said Dr. Beth Wallace, a rheumatologist at Michigan Medicine at the University of Michigan, who served as lead author on the JAMA paper. 

These conditions complicate a situation that is already difficult to interpret, making antibody tests particularly problematic, said Dr. Jeffrey Curtis, a rheumatologist and epidemiologist in Birmingham, Alabama who oversees a task force on COVID-19 vaccine guidance for the American College of Rheumatology. The task force recently stated that no routine antibody testing should be performed for fully vaccinated patients with rheumatic illnesses, such as rheumatoid arthritis, lupus or lyme disease.

“It could be falsely reassuring or falsely alarming, depending on what it shows,” Curtis said. 

In a recent small study, Dr. Ghady Haidar, of the University of Pittsburgh Medical Center, examined the antibody response of  of 67 patients at a nursing home facility who had cancers in their blood, bones and lymph nodes. 

He found that about half developed detectable antibodies a few weeks after they were vaccinated for COVID-19. Nearly half did not. Further study is needed to understand how protected these populations are, he said. It is important to remember that anecdotes are not evidence, Haidar said. So far, there is not enough data to support recommendations. 

Experts agree people with compromised immune systems should still get vaccinated, Goodman said, but they also should continue to wear face masks, practice social distancing and “remain pretty careful after vaccination, regardless of my antibody level.” That advice may be “frustrating,” Goodman said, especially when so many vaccinated people are encouraged to forgo masks and precautions, but too much is at stake. 

“It’s an area right now where there’s more questions than answers,” he said. “Unfortunately, this is a group of patients who has to wait a little longer.”

In the meantime, clinicians are testing different approaches to boost patients’ immune response. In some cases, cancer patients are scheduling their treatment therapy weeks after they get vaccinated so their body has a chance to build up an immune response. 

Several studies are being conducted to understand if people with compromised immune systems can generate a stronger response through a series of booster shots, by mixing and matching mRNA and adenovirus vector vaccines, or by administering monoclonal antibodies, which are used to treat coronavirus infection when detected early. These study results are expected to emerge in the coming months, Goodman said. 

The bottom line on antibody testing

Antibody tests are a great tool to understand if you have been previously infected with coronavirus but didn’t realize it at the time. That has been the case since these tests became available early on during the pandemic. They cannot diagnose a current illness. 

If you are fully vaccinated, these tests generally are not useful in checking in on your body’s immune response. That is especially true for people who live with compromised immune systems.

Research is still underway to better harness who is at risk for a low antibody response and how those numbers can be improved upon. In the meantime, Goodman says this is a reminder of why the general population needs to get vaccinated. 

“These are people who can’t, despite everything they’re doing, fully protect themselves with a vaccine,” he said. “It’s another good thing you do when you do get vaccinated.”