A preprint posted in the medRxiv July 31st 2019 looks at this issue.  

Riemersma and others suggest that the SARS-CoV-2 variant known as the Delta is associated with a higher viral load and increased transmissibility relative to other variants, as well as partial escape from polyclonal and monoclonal antibodies.  

The emergence of the Delta variant has been associated with increasing case counts and test-positivity rates, indicative of rapid community spread.  

Since early July 2021, SARS-CoV-2 cases in the United States have increased coincident with delta SARS-CoV-2 becoming the predominant lineage nationwide.  

Understanding how and why the virus is spreading in settings where there is high vaccine coverage has important public health implications.  It is particularly important to assess whether vaccinated individuals who become infected can transmit SARS-CoV-2 to others.  

In Wisconsin, a large local contract laboratory provided SARS-CoV-2 testing for multiple local health departments.  This provided a single standard source of data using the same assay to measure virus burdens in test-positive cases.  This included providing high-volume testing in Dane County, a county with extremely high vaccine coverage.  

These PCR-based tests provide semi-quantitative information about viral load, or amount of SARS-CoV-2 RNA in respiratory specimens.  In this study, they use the viral load data to compare the amount of SARS-CoV-2 present in test-positive specimens from people who self-report their vaccine status and date of final immunisation during a period in which the Delta variant became the predominant circulating variant in Wisconsin.  

The authors found no difference in viral loads when comparing unvaccinated individuals to those who had vaccine breakthrough infections.  Furthermore, individuals with vaccine breakthrough infections frequently tested positive with viral loads consistent with the ability to shed infectious virus.  The results, while preliminary, suggested that if vaccinated individuals became infected with the Delta variant, they may be sources of SARS-CoV-2 transmission to others.

There is, however, a problem with this study; it does not compare the number of people in the vaccinated patient population who did not contract of SARS-CoV-2 and so the study does not allow for the numbers of total patients both vaccinated and unvaccinated exposed but not necessarily contracted SARS-CoV-2.

The London General Practice, the leading London doctors’ clinic in Harley Street commends the Government and its vaccination programme and encourages all those eligible to be vaccinated.

Dr Paul Ettlinger

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