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Antibody testing serves a different purpose than rapid antigen testing

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I am writing in response to Toni Kellers’ interesting letter (Dec. 30) about the possible utility of antibody testing in place of rapid antigen COVID-19 testing.
While I understand the appeal of the concept, it has several limitations. First, one may have a protective level of circulating antibodies due to a previous COVID-19 infection, yet still become infected with SARS-CoV-2 and pass the virus on to others, particularly with the very high infectivity of the current Omicron variant.
This could have serious adverse health consequences if one believed he or she were protected by circulating antibodies, particularly among the vulnerable unvaccinated or immunocompromised.
Second, having had a previous infection does not provide perpetually high or sustained levels of protective antibodies. Data show that the short-term neutralizing antibody response is quick and persists at clinically protective levels for six to eight months (eg, humoral immunity), while longer-term protection is due to cellular immunity mediated by T cells.
In an earlier op-ed in this paper, I described that the level of vaccine-induced neutralizing antibody production needed to stimulate a clinically meaningful long-term T-cell response is above the threshold titer of 0.1 ug/ml and a median level 0.45 ug/ml. We can project that the same protective antibody levels would need to be achieved by natural infection, although this data is not yet available.

Finally, it’s understandable that there’s confusion about tests and how they differ. Antibody testing determines whether you had COVID-19 in the past and now have antibodies against the virus. A test to diagnose COVID-19 (e.g. rapid at-home antigen test or lab-based PCR molecular test) determines if you currently have the disease and may be infective to others.
To conduct an antibody test, a health care professional takes a blood sample, usually by a finger prick or by drawing blood from a vein in the arm. Then the sample is tested in a lab to determine whether you’ve developed antibodies against the virus. It is neither practical nor advisable to depend on antibody testing to determine if one is infectious to others or vulnerable to serious COVID-19 disease themselves due to levels of antibody levels below the protective threshold.
David J. Segarnick Ph.D., Upper Black Eddy


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