Dr. Matthew Binnicker, an expert in the diagnosis of infectious disease, explains the benefits and limitations of diagnostic testing as more people receive Covid-19 vaccinations.
More than 61 million Covid-19 vaccine doses have been administered in the United States since December 2020. As more people are vaccinated, this means the role of other mitigation measures, such as diagnostic testing, are starting to change.
Even as the pace of vaccine manufacturing and distribution has increased, some experts estimate that it may be mid- to late-summer before the majority of Americans have the opportunity to be vaccinated. Until then – and likely for months after – measures including masking and physical distancing will continue to be needed to prevent a resurgence in Covid-19. Diagnostic testing will remain an important tool, but it has certain limitations. Here are the answers to some common questions around Covid-19 vaccines and testing.
Could vaccination cause me to test positive for Covid-19?
In the days and weeks after Covid-19 vaccines became available, healthcare professionals observed that some vaccinated individuals tested positive by both PCR and rapid antigen tests (here’s a refresher on the different types of tests). This immediately resulted in patients and their physicians questioning whether the vaccine can result in a positive test.
The two Covid-19 vaccines (Pfizer/BioNTech and Moderna) currently authorized for use in the United States are mRNA-based vaccines. They do not consist of the entire genome of the SARS-CoV-2 virus, which causes Covid-19. Instead, these vaccines are made of a small region of the viral genome, which enters the host cell and “tricks” the cell into making a specific viral protein (i.e., the Spike protein) against which an immune response is generated. RNA molecules are inherently unstable, being targeted by enzymes that can degrade the RNA within hours. Therefore, mRNA-based vaccines will not cause a Covid-19 PCR test to be positive. Similarly, these vaccines will not cause rapid antigen tests to be positive, since the proteins produced following vaccination are not expressed in the respiratory (i.e., nasal) tract, which is sampled for Covid-19 PCR or antigen testing.
So what does it mean when a vaccinated person tests positive? It likely indicates that they were infected with SARS-CoV-2 just prior to or after being vaccinated. Although the current Covid-19 vaccines may not completely prevent an individual from being infected, they likely reduce the incidence of asymptomatic infection and have demonstrated a high level of efficacy in preventing symptomatic and severe disease.
Do I need an antibody test after vaccination to prove I’m immune?
Following vaccination, some people may question whether the vaccine was successful, and be interested in a test to determine their immune status. Certain lab tests, called serology assays, are designed to detect antibodies that are generated in response to an infection. Although these tests can provide information on whether an individual has been exposed to a virus in the past, they may not provide a reliable measure of a person’s immunity against the virus.
Serology tests for infectious diseases typically only tell us whether antibodies against a pathogen are present or absent, but don’t provide specific detail on “how much” antibody is present. Therefore, an individual may test positive for antibodies, but the level of antibodies detected by the test doesn’t always correlate with the amount required to protect them from becoming sick. Similarly, a negative antibody test does not always mean that a person lacks immunity, since other parts of the immune system, such as T cells, can play a vital role in fighting an infection. Due to these limitations, it’s unlikely that routine antibody testing after Covid-19 vaccination will be recommended.
Will we need to test for Covid-19 after the pandemic ends?
Cases of Covid-19 are on the decline in many parts of the world, likely due to a combination of strict precautionary measures, potential seasonality effects and increasing immunity from natural infection and vaccination. However, the emergence of new SARS-CoV-2 variants has given rise to the distinct possibility of another surge in Covid-19 cases. Whether or not a resurgence occurs, at some point, the pandemic will come to an end. When that happens, it remains unclear what will become of Covid-19.
Will SARS-CoV-2 disappear entirely, or will it become an endemic cause of respiratory illness? Given that SARS-CoV-2 has reached all corners of the world and will continue to mutate over time, it is most likely that Covid-19 will continue to cause outbreaks of mild- to moderate respiratory disease, potentially with a seasonal pattern similar to influenza. Therefore, testing for Covid-19 will likely continue to be important even after the pandemic has ended. Healthcare providers will need to distinguish between Covid-19, influenza and other causes of respiratory disease so that patients can be treated and managed effectively. In other words, it’s highly probable that Covid-19 will be with us – at least to some degree – for years to come.
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