Can you have coronavirus antibodies yet still be contagious?


The night after visiting the emergency room to get tested for the coronavirus, Theresa Canning Zast said, she soaked through four T-shirts with intense night sweats, and suffered chills and aches in her legs. Still, she didn’t think her early symptoms were linked to the pandemic that was just starting to ramp up in her home state of New York.

“It was not in my chest,” Canning Zast told Yahoo News. “It was flu-like, so I actually didn’t think it was COVID. I just went to the emergency room as a precaution.

“I got the call that I was positive, and I was kind of shocked.”

After weeks of self-isolating, with her symptoms improved, Canning Zast assumed she had recovered and decided to donate her plasma to be used as convalescent therapy — a technique that has been used by doctors to treat infectious diseases for over a century, and is now being used to help patients with acute cases of COVID-19.

But on April 7, more than three weeks after initially testing positive for the coronavirus, Canning Zast got another surprise: A blood test showed that her immune system had developed enough antibodies against the virus to qualify for plasma donation, but she would be unable to donate because her nasal swab test had yet again come back positive for the coronavirus.

It would be another week before Canning Zast finally tested negative for the virus, on April 15.

“That was a huge relief,” she said of the results. “It had freaked me out that I was positive for so long after.”

A donor laying on a transfusion chair while wearing a protective mask during the blood donation. Thai Red Cross has requested blood plasma donations from patients recovering from COVID-19. (Photo by Amphol Thongmueangluang/SOPA Images/LightRocket via Getty Images)
A donor giving blood. (Amphol Thongmueangluang/SOPA Images/LightRocket via Getty Images)

As Americans are learning, there are two kinds of coronavirus tests that serve different purposes: the nasal swab test, which detects an ongoing infection — the one that keeps coming up at briefings of the White House coronavirus task force — and a blood test for the presence of antibodies, which provides evidence of past infection. The blood test can serve two purposes: to qualify the person tested for donating blood plasma to treat other patients, and to determine if the person is now immune from reinfection.

But there is enormous uncertainty about the second part.

The Centers for Disease Control and Prevention currently advises that people with COVID-19 can end home isolation at least seven days after symptoms first appeared, provided their symptoms have improved and they have gone without fever (and without fever-suppressing medication) for at least 72 hours. Yet as Canning Zast’s experience exemplifies, an alleviation of symptoms or even the presence of antibodies is not enough to ensure that the virus has fully left a person’s system. And questions still remain about how long it takes to be completely rid of the virus, and whether that time frame correlates with how long a person is contagious.

“When you ramp up your antibodies, you also have to come down on the virus. And the question is, can people have antibodies but also still have the virus in their respiratory system?” said Dr. Dara Kass, a Yahoo News medical contributor and associate professor of emergency medicine at Columbia University Irving Medical Center. “That is a question we’ve been asking since day one. And it seems like the answer is yes, you can still have virus in your respiratory system but also have antibodies to the virus in your blood.”

An antibody is a protective protein produced by the immune system, which binds to foreign substances such as viruses so they can be neutralized and removed from the body. Testing for antibodies to the coronavirus has been ramping up in parallel with a gradual — albeit slow — increase in nasal swab testing for the coronavirus itself.

A COVID-19 blood test is administered outside of Delmont Medical Care on April 22, 2020 in Franklin Square, New York.  The test identifies antibodies to the coronavirus. (Photo by Al Bello/Getty Images)
A COVID-19 blood test is administered on Wednesday in Franklin Square, N.Y. (Al Bello/Getty Images)

The risk of spreading the coronavirus by carriers who don’t, or don’t yet, show symptoms was the impetus behind stay-at-home orders. Now that more people have recovered, uncertainty about how long the virus stays in the body or how long carriers are contagious represents a different challenge: knowing when it’s safe for people who do have detectable antibodies to resume social contact.

“We don’t know if they find virus in your system if you’re still really contagious,” Kass told Yahoo News. “It’s just the idea that you can’t be contagious is off the table.

“The longest anyone has really been contagious with this is probably about five weeks, based on the data we have,” she explained. “And so it’s unlikely that most people who have antibodies are still contagious. But until we have better data we have to act with caution.”

Kass, an emergency room physician in New York City, tested positive for the coronavirus in mid-March. Although she was able to return to work weeks later, Kass continued to stay in a hotel room after her ER shifts. Until she was able to acquire another coronavirus test, she was nervous about returning home and possibly exposing her family to the virus — particularly her son, who had a liver transplant and consequently has a compromised immune system.

“I waited because I knew that having antibodies wasn’t perfectly correlated with the ability to not expose my family,” Kass said.

“For me, in my family, the risk of spreading the virus was so significant that even if I had antibodies, I still couldn’t come home until I tested negative for the virus. When I tested negative for the virus, I moved back home.”

Further complicating the picture are questions about the significance of the antibodies themselves. Doctors believe harvesting antibodies from recovered patients to augment the immune system in those currently infected is a promising approach. But the presence of antibodies in the blood doesn’t necessarily confer complete immunity. And even if they do offer some protection, it’s uncertain how long it lasts. Researchers have established a threshold for donating plasma for therapy: an antibody concentration of at least one part in 320. But it isn’t known what level in the bloodstream actually is necessary to prevent infection.

“There’s a lot of work being done on whether or not a certain level of antibodies actually does correlate with immunity, but we don’t know that yet,” Kass said.

While testing negative for the coronavirus, following a positive test, can provide some peace of mind, the testing system in the United States isn’t robust enough for everyone who tests positive to also receive a second test confirming that they no longer have the virus. Only 4.9 million tests have so far been conducted in the U.S., according to Johns Hopkins University. And Kass said that for now, simply identifying who has the virus is more important than retesting recovered patients.

“We don’t have the capacity right now to screen test and also screen people back to negative before they go back into their community. We may have that later on as we scale up testing and have fewer cases,” she said.

“The most important thing we need to do in all the cities in America and every state in America is figure out where the infection is so that we can keep track and trace and isolate those communities away from spreading it to other people. That’s definitively the priority.

“Everything else, whether it’s antibody testing or testing negative, becomes almost secondary to that.”


Click here for the latest coronavirus news and updates. According to experts, people over 60 and those who are immunocompromised continue to be the most at risk. If you have questions, please refer to the CDC’s and WHO’s resource guides.

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