14 Apr 2021
COVID-19 Testing of Vaccinated Individuals
While the new COVID-19 vaccines are successful at preventing the COVID-19 syndrome, it is still unknown if the vaccines prevent people from getting infected and passing on the SARS-CoV-2 virus.
Vaccinated People Can Be Asymptomatic Carriers
Preclinical studies of COVID-19 vaccines demonstrated persistent virus in nasal swabs even after being vaccinated. During the trial of Moderna’s vaccine, researchers saw a significant drop but not elimination in the number of asymptomatic infections among people who received the first shot of the two-dose vaccine, compared with those who received a placebo1. During a trial of the AstraZeneca vaccine, researchers swabbed participants every week, and estimated a 49.3% reduction in asymptomatic infections among a subset of vaccinated participants compared with the unvaccinated group2. This data suggests people can have asymptomatic infections even after being vaccinated.
Viral Load is Reduced in Vaccinated Individuals
Researchers have noted a significant drop in viral load in people who were infected with SARS-CoV-2 in two to four weeks after receiving their first dose of the Pfizer vaccine, compared with those infected in the first two weeks after the injection3. The AstraZeneca trial also observed a larger reduction in viral load in a group of vaccinated participants than in the unvaccinated group4.
Low Limits of Detection Critical for Detection of Samples with Low Viral Loads
If the viral load of a sample is lower than the LOD or cut-off value of a real-time RT-PCR assay, a false negative result will be obtained. Each 10-fold increase in the LoD of a COVID-19 viral diagnostic test is expected to increase the false negative rate by 13%5. Therefore, an assay with low LOD (high sensitivity) is critical for accurate detection of samples with low viral load.
Vaccinations Reduce, but Do Not Eliminate the Chance of Viral Spread
CDC states that while the risks of SARS-CoV-2 infection in fully vaccinated individuals and transmission to others is reduced, it cannot be eliminated in the setting of continued widespread community transmission of the virus. Vaccinated people could potentially still become infected and spread the virus to others6.
Testing Vaccinated Individuals for Asymptomatic Infections
Since vaccinated individuals have a reduced viral load, to check to see if they are asymptomatic carriers, a highly sensitive test with a low LOD must be used to prevent the reporting of false negatives. The FDA developed an experiment to precisely compare the performance of the nucleic acid-based SARS-CoV-2 assays which have received EUA authorization and published a comparative performance analysis. This assessment used the FDA SARS-CoV-2 Reference Panel which allowed a consistent determination of the relative sensitivity of these tests. See how the assays rank in comparison to each other. Recently obtained Next Generation Sequencing (NGS) data on vaccinated individuals who tested positive with the PerkinElmer New Coronavirus Nucleic Acid Detection Kit provided orthogonal confirmation that the virus was present in these individuals.
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