Pathologists Answers

There are several reasons for a COVID-19 antibody test to be negative following vaccination. This is one of the reasons why routine antibody testing post-vaccination is not recommended.

A negative antibody test following vaccination could be due to: Incorrect test requested. Currently licensed vaccines contain spike protein in some form or another. As such, an S-antibody (antibodies to spike protein) assay should be requested. Many assays only test of N-antibody (antibodies to nucleocapsid protein), which will not detect vaccine response for currently licensed vaccines.
Test performed too early. It may take several weeks before antibodies become detectable. Testing too soon after vaccination can result in negative antibody tests. This may be resolved by testing at a later time point.
Low sensitivity test. Some assays, such as rapid lateral flow tests, have a low sensitivity and may not detect low levels of antibody.
Heterogenous seroconversion response. Not all patients produce a detectable antibody response, and in those who do seroconvert, the timing of said seroconversion varies. Negative antibody tests post-vaccination do not necessarily imply vaccine failure, as these tests do not measure cellular immunity, which may be present even in the absence of detectable antibodies. The exact correlate of protection is not known.

Dr Howard Newman Clinical Virologist Pathcare Vermaak

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The NICD recommends that symptomatic patients with mild disease can de-isolate after 10 days provided their fever has resolved and their other symptoms are improving. Full recovery may take longer and symptoms such as tiredness, cough and loss of smell may persist for several weeks. If fever persists or other symptoms are not improving by day 10, a longer isolation period may be needed in consultation with a health care practitioner.

 

Dr Dominique Goedhals, Virologist Pathcare Vermaak

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