Should Patients be Tested After COVID19 Vaccination?
Vaccination is not expected to result in positive diagnostic tests. While both the Pfizer-BioNTech and Moderna vaccines include mRNA corresponding to the spike protein, the quantity of mRNA in each dose is very small, much less than is generated during SARS-CoV-2 infection. Also, the mRNA in the vaccine does not amplify like the virus would. Most PCR tests target other portions of the SARS-CoV-2 genome, and in combination with low quantity of mRNA, it is unlikely that an individual would test positive due to the vaccine.
Serological tests identify the presence of antibodies directed towards a particular protein of the SARS-CoV-2 virus. The Pfizer/BioNTech and Moderna vaccines generate antibodies against the spike protein, so only serological tests that detect those antibodies would yield positive results after vaccination. Conversely, tests that target antibodies against the nucleocapsid protein would not detect antibodies induced by the vaccine or provide any indication if the vaccine successfully generated an immune response.
Another important question is whether there is an immediate indication for spike antibody testing post-vaccination. According to both Pfizer and Moderna trials, everyone vaccinated develops IgG antibodies capable of reducing symptomatic infection within 15 days after first dose. Vaccination produces higher antibody levels than natural infection. It is not known how quickly antibodies will wane after vaccination, but antibodies produced after natural infection last at least 8 to 9 months. For these reasons, there does not appear to be an immediate indication for spike antibody testing post-vaccination.
There is probably only one indication for nucleocapsid antibody testing: That is to diagnose natural infection either prior to after an individual has been vaccinated with mRNA or vector-based vaccines coding for spike protein.
Reference
Johns Hopkins COVID-19 newsletter, February 5, 2021
