As antibody tests for the coronavirus become more widespread, a major question still remains: Does a positive test for antibodies mean that a person has developed immunity to the virus?
It may be months before the answer is clear, but that doesn't mean the tests can't yield valuable public health information in the meantime.
Antibody tests provide information about one thing: whether a person's immune system has developed antibodies needed to fight off a particular infection. A positive test means a person has antibodies, and therefore had the virus at some point; a negative test means a person does not have antibodies and has not yet contracted the virus.
By showing how many people haven’t been infected, the tests provide a picture of who remains vulnerable to the coronavirus.
"There still are people that this virus can infect," Dr. Maria Van Kerkhove, head of emerging diseases and zoonosis unit at the World Health Organization, said during a media briefing Monday. "It's important we remain vigilant."
What remains unclear, however, is whether coronavirus antibodies work well enough, or last long enough, to protect a person from being reinfected.
On Saturday, the WHO caused a bit of global angst among people who are depending on those antibody tests to prove their immunity status — perhaps giving them a pass to return to work or regular social activity — by tweeting that there is not enough evidence that people who have recovered from COVID-19 are protected from reinfection.
The tweet has since been deleted, though the WHO provided a link to the archived thread.
On Monday, the agency clarified what it meant by the original tweet.
"Just because there's no evidence in this area doesn't mean that there's no immunity," Van Kerkhove, said. "It just means that the studies haven't been done yet."
Outside experts agreed. "It's very important to not assume that a positive antibody test means that you're definitely immune, and that you couldn't become infected or that you couldn't spread infection to somebody else," Dr. Marybeth Sexton, an assistant professor of infectious diseases at Emory University School of Medicine, said.
"So all of the general principles we've been talking about — washing your hands, not going out when you're sick, wearing a face mask in public — those things all still apply regardless of the antibody test result you get," Sexton told NBC News.
Even if it turns out that a person has some immunity, it's unclear whether it would last weeks, months or even into next flu season. Only time will tell.
"It could be that somebody who was sick last month in March now has antibodies," Dr. Daniel Kuritzkes, head of the division of infectious diseases at Brigham and Women's Hospital in Boston, said. "By October, it's possible that those antibodies could be declining and therefore less protective."
Many remain vulnerable
While the implications of a positive COVID-19 antibody test remain uncertain, a negative test may give public health officials a better understanding of which populations remain at risk. People who have not been infected with the new coronavirus are susceptible because the virus has never been seen before in humans.
Preliminary antibody test studies suggest the vast majority of Americans have not developed COVID-19 antibodies, and therefore have not been infected.
In the hardest-hit state in the country, New York, preliminary study results found that about 15 percent of residents tested positive for antibodies, according to Gov. Andrew Cuomo. An early estimate of residents in Los Angeles County suggested that just 4 percent had COVID-19 antibodies, meaning 96 percent apparently did not.
That kind of information could be particularly useful as the U.S. enters flu season this fall. The WHO is closely monitoring the Southern Hemisphere now, where flu season is imminent.
How countries in the Southern Hemisphere cope with both viruses circulating at the same time "will greatly benefit countries in the Northern Hemisphere, who may face the same situation in six months' time," Dr. Mike Ryan, executive director of the WHO's health emergencies program, said during Monday's briefing.
Some populations or areas of the country may need different social distancing measures as time goes on, depending on their COVID-19 vulnerability.
Not all antibodies are created equal
Much more research is needed to determine which antibodies may offer the best protection against COVID-19. In general, there are five different types of antibodies, some of which are more protective than others.
The Mayo Clinic in Rochester, Minnesota, has been performing COVID-19 antibody testing and studying the results. Thus far, it appears that the severity of illness may play a role in which kinds of antibodies are created.
"We are focusing on what are called IgG antibodies," Elitza Theel, the clinic's director of the infectious diseases serology laboratory, said.
IgG antibodies appear to be longer-lasting, Theel said, and, based on research from other infections, are typically associated with future protection.
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Theel's team has found that it's mostly the sickest patients — those who've been hospitalized — who produce IgG antibodies. And it appears that a small percentage of patients with milder cases of the disease aren't making the robust IgG antibodies.
"This is very preliminary," Theel warned. "But there might be a differential immune response between very sick individuals and individuals who have a more mild course of disease."
Experts say it's likely to take months to answer all of the questions surrounding immunity to COVID-19.
"This is nowhere near over," Van Kerkhove said. "We need everyone to be mentally prepared that we have some more to go."