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Similar viral load in vaccinated and unvaccinated persons with delta variant infection

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eMediNexus    03 August 2021

The delta (B.1.617.2) SARS CoV-2 variant produces similar viral loads in the vaccinated as well as the unvaccinated individuals, if they acquire the infection, suggests a new study from the US Centers for Disease Control and Prevention (CDC).

The study published in the latest Morbidity and Mortality Weekly Report examined a cluster of Covid-19 cases in 469 Massachusetts residents, who developed the infection in an outbreak in July in Barnstable County in Massachusetts following several events and large public gatherings. These cases had travelled to the town during July 3–17. No deaths were reported. Of these, 346 (74%) were breakthrough infections in fully vaccinated persons. Of these, 79% developed symptoms.

Genomic sequencing identified the delta variant in 90% of samples from 133 patients. The cycle threshold (Ct) values on RT PCR in specimens from 127 vaccinated persons with breakthrough infections (Ct=22.77) was similar to those from 84 persons who were unvaccinated, not fully vaccinated, or whose vaccination status not known (Ct=21.54) suggesting that the vaccinated and unvaccinated persons infected with SARS-CoV-2 also had similar viral loads.

High viral load is associated with an increased risk of transmission of infection. A Nature Communications report published in October last year had said that SARS-CoV-2 viral load is also associated with increased disease severity and mortality (Nat Commun. 2020 Oct 30;11(1):5493).

The delta variant is highly transmissible. Only last week, the CDC had reportedly described the delta variant to be “as contagious as chickenpox and far more contagious than the common cold, Spanish flu, smallpox, Ebola, MERS and SARS. Only measles appears to spread faster than the variant”. Lead WHO epidemiologist and COVID-19 technical lead, Dr Maria Van Kerkhove called Delta “dangerous and the most transmissible SARS-CoV-2 virus to date”. It has now spread to 132 countries and has become the predominant strain now. Several countries across the world are seeing a marked surge in cases, largely driven by the delta variant.

A medRxiv preprint published earlier this month had shown that people infected with the delta variant had ~1000 times viral load higher on the day they were firstly detected than in persons with the earlier strains.

Following the publication of this study, there is an apprehension that unlike with other variants, vaccinated people infected with Delta can also transmit the virus. Basis these findings and to prevent transmission of virus to others, the CDC has updated its masking guidance and recommended that all persons, including those who are fully vaccinated, should wear masks in indoor public settings in areas where transmission of the infection is high or substantial or if they are immunocompromised or at increased risk for severe disease from COVID-19, or if they have someone in their household who is immunocompromised, at increased risk of severe disease or not fully vaccinated.

The pandemic is far from being over. All covid preventive measures must be followed. Any complacency or irresponsible behavior on our part will surely invite the third wave.

Source: Brown CM, et al. Outbreak of SARS-CoV-2 infections, including COVID-19 vaccine breakthrough infections, associated with large public gatherings — Barnstable County, Massachusetts, July 2021. MMWR Morb Mortal Wkly Rep. ePub: 30 July 2021. DOI:http://dx.doi.org/10.15585/mmwr.mm7031e2.

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