The human body produces antibodies when it fights an infection. This process helps it build immunity to that specific infection. An example of a virus that causes an infection, is the new coronavirus, which causes COVID-19.
Antibody testing helps determine if a person has had the new coronavirus, or SARS-CoV-19, by checking for the presence of antibodies.
Read on to learn more about how the body develops COVID-19 antibodies and antibody testing.
Antibodies are special Y-shaped proteins. Their role is to recognize antigens, or foreign particles, present in microbes, such as viruses.
Each antibody specifically recognizes a particular antigen associated with a particular microbe, similar to a lock and key mechanism.
Immune cells called lymphocytes are responsible for recognizing and responding to antigens.
B lymphocytes, a type of lymphocyte, produce antibodies, which then bind to the antigen. This action marks the antigen for destruction.
However, the antibodies cannot penetrate the target cell, so another type of lymphocyte called T lymphocytes must destroy the microbes.
After the initial infection, the B lymphocytes will recognize the antigen. So, if the body encounters that antigen again, the B lymphocytes are ready to make antibodies at a faster rate, preventing reinfection.
COVID-19 antibody tests primarily detect the following types of antibodies:
- IgM antibodies, which develop early in an infection.
- IgG antibodies, which develop a few weeks postinfection and are key in establishing immunity after infection.
- IgA antibodies, which protect the surfaces of mucous membranes throughout the body.
Coronavirus antibody tests also check for the total antibody count.
A coronavirus antibody test checks specifically for the presence of antibodies to the novel coronavirus, SARS-CoV-2.
A positive antibody test indicates that the person has had COVID-19 in the past. It does not mean they currently have a coronavirus infection.
How long does it take to detect COVID-19 antibodies?
In the case of COVID-19, it takes approximately 1–2 weeks after the onset of symptoms for people to develop detectable antibodies in the blood.
A 2020 study in Nature Medicine reports that, of the 285 study participants who required hospitalization for severe COVID-19, all developed SARS-CoV-2 specific antibodies within 19 days of symptom onset.
Do COVID-19 antibodies protect against reinfection?
The Centers for Disease Control and Prevention (CDC) advise that there is not yet enough evidence to say whether the presence of such antibodies provides immunity and protects that person from reinfection.
One preliminary animal study on monkeys reports no reinfection among the animals that researchers reexposed to the novel coronavirus nearly a month after the first infection.
While these findings are promising, the data is limited, and scientists do not know how long immunity will last. Further research may shed more light on this.
In the case of other coronaviruses, such as MERS-CoV and SARS-CoV, antibodies may remain detectable for up to 3 years postinfection.
According to the CDC, the tests target antibodies against two main antigens of the SARS-CoV-2 virus:
- Spike glycoprotein (S), which is present on the surface of the virus. It is a crucial protein that helps the virus fuse with cells in the body, so it can infect them.
- Nucleocapsid phosphoprotein (N), which is present in the virus in greater quantities than S protein. This protein helps the virus replicate itself within the cells it infects, helping it to proliferate throughout the body.
The CDC state there are two broad classes of antibody testing for COVID-19:
Binding antibody tests
Binding tests do not use a live virus to detect antibodies. Instead, they use purified SARS-CoV-2 proteins.
These tests can show results in as little as 30 minutes. There are two categories of tests that detect binding antibodies:
- Point-of-care (POC) tests, which require only a finger prick blood sample.
- Laboratory tests that require the use of trained laboratory staff and specialized instruments.
Neutralizing antibody tests
Neutralizing antibody tests determine the ability of the antibodies to prevent viral infection in a test tube. This type of test involves mixing a blood sample with live virus, and trained technicians carry it out in a laboratory.
The Food and Drug Administration (FDA) have not yet authorized the use of neutralization tests for SARS-CoV-2.
In most cases, when an individual tests positive for COVID-19 antibodies, it indicates that they have already had the virus.
The CDC suggest that people who test positive for antibodies may continue with their normal activities only if they are symptom-free and have not been in contact with a known COVID-19 case. They must still take steps to protect themselves and others.
Antibody test results should not determine if someone can return to work, or if they can congregate with others in schools and other settings. It is important to take into account a person’s symptoms and their exposure to others with the virus.
Those who receive positive results but have symptoms or who have been in contact with a known COVID-19 case should self-isolate. They should also take additional steps to protect themselves and others.
In either case, people should maintain physical distancing, wash their hands often, and wear a cloth face covering when in public.
What is a false positive result?
In some cases, the result may be a false positive. A false positive means that the person does not have the antibodies, despite receiving a positive result.
The CDC mention some people may receive a false positive because the antibody tests for SARS-CoV-2 are not 100% accurate.
The FDA advise that, occasionally, a second antibody test may be necessary to get accurate results.
Once accurate antibody testing becomes widely available, it may help limit the spread of the novel coronavirus.
Accurate results will indicate how many people have recovered from COVID-19, even if they did not have symptoms. This may help determine who has some level of immunity to the virus.
Accurate test results can also help with contact tracing of people who have interacted with those with the virus.
Determination of the overall spread of the disease will also be possible with antibody testing.
Another benefit is that people who have had COVID-19 may be able to donate plasma that contains antibodies to the virus. The plasma may help treat people with severe COVID-19 disease. This treatment is known as convalescent plasma therapy.
There are some limitations of COVID-19 antibody testing. According to the American Society for Microbiology, antibody testing:
- cannot definitively determine if a person has immunity
- cannot determine current or very recent cases of COVID-19
- must not inform a person’s physical distancing, hygiene practices, or personal protective equipment (PPE) use
Similarly, the CDC advise that antibody test results should not determine when people can return to work or mix with others. Neither should it affect when they can group together in schools, correctional facilities, and other settings.
These limitations may reduce once more accurate and widespread testing becomes available, and researchers have carried out more studies.
The available research indicates that people who have had COVID-19 develop antibodies to the novel coronavirus. However, it is not yet clear if these antibodies confer immunity. Even if they do provide immunity, it is not clear how long this immunity would last.
Individuals must, therefore, continue to take precautions to protect themselves and others from the spread of COVID-19, even if they test positive for antibodies to the disease.
Hopefully, advances in research and testing methods will shed more light on the potential benefits of COVID-19 antibodies and antibody testing practices.