How protected are we against Covid-19? Scientists search for a test to measure immunity
Millions of Americans are wondering about their protection after a winter of booster shots and Omicron infections. As mask mandates are lifted and restrictions removed in a step toward normalcy, a test to measure immunity would be a powerful tool to measure individual risk.
About 95% of Americans 16 and older have antibodies against Covid-19 as of December, the most recent date that data is available. It's one thing to measure antibody levels. It's another to measure how much they protect you against Covid-19.
The US Federal Drug Administration recommends against checking antibody levels at all because there's no agreed-upon way to calculate how any given antibody level protects you from infection or severe disease. It may also give a false sense of security.
Scientists are trying to fill the knowledge gap. Studies measure average antibody levels across a population to check vaccine efficacy, often using antibody levels months after vaccination to determine the need for a booster. One study in people who got the Moderna vaccine found that higher antibody levels after vaccination were associated with lower risk of Covid-19 infection.
Tests to measure antibodies can be either quantitative or qualitative. Quantitative tests provide a specific number, up to a certain point, of antibodies in the blood. Qualitative tests will indicate only whether certain antibodies were detected. Results of qualitative tests are either positive, negative or indeterminate for neutralizing antibodies.
When it comes to measuring neutralizing antibodies specifically, there is only one type of test that has been given emergency FDA authorization to detect them, and it is qualitative.
Several studies have demonstrated that neutralizing antibodies are a strong correlate for protection against symptomatic infection with Covid-19 and its variants, with boosters enhancing neutralization.
However, reaching conclusions based on one person's antibody levels is far more limited. Gilbert compares individual antibody titers with a dipstick for oil in a car.
There's a key difference in how antibody levels compare in people who have been vaccinated against Covid-19 versus those who have been infected with the coronavirus.
Observing levels of immune cells and antibodies as they change over time in different groups of people can help scientists learn how to create vaccines and time vaccinations to replicate the strength of hybrid immunity without the actual infections.
Antibody levels are only one part of the immunity story. There are also T cells, a type of white blood cell that helps fight infection by killing cells that have been infected with a virus or by helping another type of white blood cell, the B cell, create antibodies.
Unlike antibodies, which are responsible for preventing an infection, T cells are responsible for destroying cells that are already infected. T cells may play a big role in vaccine efficacy against serious disease from more transmissible coronavirus variants, such as Omicron.
The Omicron surge has demonstrated that infection rates can still be high even after a vaccine or a prior infection. With the possibility of more variants forming, Covid-19 has become a moving target requiring a constantly evolving understanding of these correlates of protection.
Ahmed, Tasnim. “How Protected Are We against Covid-19? Scientists Search for a Test to Measure Immunity.” CNN, Cable News Network, 22 Mar. 2022, https://www.cnn.com/2022/03/22/health/measuring-immunity-correlates-of-protection/index.html.
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