Feature Article: Antibody Testing After COVID-19 Vaccination
Published on in Parents PACK
Published on in Parents PACK
As people continue to get vaccinated, some wonder how to know if the vaccine worked. Answering this question is more complex than it may appear. So, let’s take a look.
For some pathogens, scientists have identified a “correlate of protection,” which is typically a quantity of specific antibodies above which a person is protected against an infection and below which protection is uncertain. For example, if a person has antibodies against hepatitis B surface protein of at least 10 milli-international units per milliliter of blood (10 mIU/mL), they are considered immune to hepatitis B. On the other hand, if they have less than 5 mIU/mL, they are considered susceptible to hepatitis B infection. For those with levels between 5 mIU/mL and 10 mIU/mL, their immunity is uncertain. Healthcare providers use our understanding of these measures to inform the diagnosis, treatment and prevention of hepatitis B in their patients.
Unfortunately, correlates of protection have not been identified for all pathogens, including SARS-CoV-2, the virus that causes COVID-19. Scientists are working to identify such a correlate, and early studies suggest that particular antibodies against the virus may serve this role. However, more research is needed to reproduce these findings, better understand which specific antibodies indicate protection, and determine at what antibody levels protection is likely. It’s also important to note that the immune system is remarkably compensatory, meaning there may be more than one path to protection. In the case of COVID-19, this is evidenced by some data suggesting that certain types of T cells can also affect a person’s course of infection. Whether T cells can prevent infection, or whether they just alter severity of infection, remains to be determined. With this in mind, a single correlate of protection may only tell part of the story when it comes to an individual’s immunity against COVID-19.
So, now you may be wondering, if we don’t have a correlate of protection, how do we know COVID-19 vaccines work? By comparing the infection rates of trial participants in the vaccine and placebo groups, scientists were able to determine that fewer people got infected after vaccination. This example demonstrates an important distinction between evaluating groups of people versus individuals:
Defined correlates of protection would also allow future vaccine studies to be designed differently. These correlates obviate the need for large clinical trials in people who did or didn’t receive the vaccine. For example, with a correlate of protection, scientists could vaccinate a group of individuals with a potential new vaccine and know that if their immune responses reached a certain level, the vaccine worked. Likewise, scientists may also compare the immune responses following the new vaccine with those of a group of individuals who got an existing vaccine to identify differences in their effectiveness. Efforts to identify a correlate of protection against COVID-19 are important and ongoing.
Because we do not yet have a correlate of protection, getting an antibody test to see if the vaccine worked is not as helpful as it would appear. Additionally, all antibody tests are not created equal, so some individuals have reported negative antibody tests after being vaccinated and are concerned that the vaccine did not work. Even if the vaccine worked, a vaccinated individual may get a negative result for two reasons:
Because we do not know what antibody tests mean in terms of protection and using inappropriate or low-quality tests wastes resources, antibody testing to check whether a COVID-19 vaccine worked is not recommended. Further if someone is tested, the results should be interpreted with caution.
As you may now realize, telling whether one’s COVID-19 vaccine worked is not as straightforward as it may seem. So, people wonder what they should do, particularly because uncertainty is uncomfortable.
As communities reach herd immunity, even people who are not protected will be able to move about more freely and forgo some of the public health measures that we continue to practice.
If you are interested in learning more about any of these ideas, check out these resources:
Categories: Parents PACK May 2021, Feature Article
Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.
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