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Jan 27, 2022
6 min read

COVID antibody tests: what do they mean?

Your immune system makes antibodies after infection or vaccination. Some studies suggest that higher levels of antibodies are more protective against future disease, but when it comes to COVID-19, nobody knows the precise antibody level for immunity against infection. At this time, the FDA does not recommend routine testing for antibodies against COVID.

Important

Information about the novel coronavirus (the virus that causes COVID-19) is constantly evolving. We will refresh our novel coronavirus content periodically based on newly published peer-reviewed findings to which we have access. For the most reliable and up-to-date information, please visit the CDC website or the WHO’s advice for the public.

When you’re exposed to a virus or vaccine, your immune system begins generating protective molecules called antibodies. These antibodies remain in your system long after the infection is gone and continue to protect you against re-infection. 

But the correlation isn’t as clear as we might like: antibody tests can reveal the presence of antibodies that indicate infection or vaccination but don’t necessarily protect you from re-infection. Even if you do specifically look at protective antibodies (known as neutralizing antibodies) the threshold for what level of antibodies might grant immunity remains unclear. 

For that reason, the Centers for Disease Control and Prevention (CDC) does not currently recommend routine antibody testing (CDC, 2021).

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Antibodies after infection 

Most studies demonstrate that COVID antibodies are seen about two weeks after infection (Post, 2020). 

Even if you never had symptoms, you’ll still develop antibodies. A study of over 300 people with COVID found that the antibody titer was similar in those with no symptoms and those with mild to moderate COVID symptoms (Jonsdottir, 2021). 

Research shows that the antibodies from a previous infection are likely to protect you from another infection for several months. The antibody level decreases over time, but studies show that your immune system retains something called memory B cells, which can make new antibodies to protect you from future exposure (Gaebler, 2021; Wheatley, 2021). 

Antibodies after the COVID vaccine

The COVID vaccine trains your immune system to recognize the spike proteins present on the outside of the virus so it can fight the coronavirus if you’re exposed. Your immune system identifies the proteins as foreign and makes antibodies against them. These antibodies can protect you against infection and severe disease. 

There are different types of antibodies and tests to differentiate between them. People who’ve had the vaccine but were never infected with coronavirus will only have antibodies against the spike protein. Those who’ve been infected will have antibodies against the spike protein and other components of the virus (like the outer shell). 

A healthcare provider may decide to use an antibody test to determine whether or not you were infected in the past.

Is there a protective antibody titer?

How do you know if your immune system made enough antibodies to protect you? Clinical trials have shown that certain antibodies are protective against severe disease and hospitalization. However, they have not identified the specific level needed for immunity or how long the antibodies protect you.

One study suggested that higher antibody titers (the level of antibodies in the blood) were associated with better protection. It also showed that COVID vaccines produced higher titers than a mild infection (Bartsch, 2021). 

A second study also found that higher levels of neutralizing antibodies, which are a specific type of antibody, are more protective than other types of antibodies for longer periods. But it’s not clear how long that protection lasts. It also notes that immunity will wane and boosters might be required (Khoury, 2021).

Other studies point out that while the immune system has a robust response to the vaccine and generates high titers of antibodies, the levels drops quickly. Your body has other immune system defenses (like the protective lining of your nose and mouth and trained memory cells) that a titer does not measure. These factors make it difficult to tell if you are immune to infection based just on an antibody level (Sui, 2021).

Boosters

Antibodies don’t last forever, and your antibody titer will drop over time. Your immune system is trained to respond to the same invaders, but researchers need more data to see how effective this response will be. 

In addition, COVID variants (like Delta and Omicron) continue to arise. Many variants have mutations that help them escape your immune system because the antibodies you have are trained against elements of the virus that are different in variants. Boosters can increase your antibody titer, and vaccines are being developed to specifically target variants of concern. 

Based on recent data, the CDC recommends a booster shot to bolster your immune response. The recommendation is the same even if you have a positive antibody test (CDC, 2021).

Based on the available information, the U.S. Food and Drug Administration (FDA) does not recommend testing antibody titers. Detecting antibodies in the blood shows that you had a past infection or a vaccine, but nobody knows the magic number that guarantees immunity (FDA, 2021). 

Having antibodies does not mean you should stop wearing a mask or following other safety precautions. Guidelines might change in the future when more information becomes available, but for now, COVID antibody tests are not routinely recommended.

References

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