Similar viral load in vaccinated and unvaccinated individuals infected with SARS-CoV-2 Delta variant

The causative agent of the coronavirus disease 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), is an extremely virulent single-stranded RNA virus. Several COVID-19 vaccines have received emergency use authorization (EUA) from several global regulatory bodies, and subsequently, vaccination programs have been initiated in many countries.

Study: No Significant Difference in Viral Load Between Vaccinated and Unvaccinated, Asymptomatic and Symptomatic Groups Infected with SARS-CoV-2 Delta Variant. Image Credit: Kirill Kamionskiy/ Shutterstock

Scientists have stated that vaccination reduces the mortality rate and severity of disease caused by SARS-CoV-2. However, breakthrough cases are now being reported owing to the emergence of SARS-CoV-2 variants. Some of the variants are capable of evading the immune protection induced via vaccination or natural infection.

To date, all the available vaccines have been developed against the spike protein of the original SARS-CoV-2 strain. The evasion of immune responses by the SARS-CoV-2 variants, especially the Delta variant, has questioned the effectiveness of the available COVID-19 vaccines.

Are vaccinated individuals protected from SARS-CoV-2 variants of concern?

Many studies have indicated no difference in cycle threshold values (Ct-values) between vaccinated and unvaccinated individuals. However, others have contradicted these results and claimed that breakthrough infections, especially among asymptomatic individuals, show a lower viral load than unvaccinated individuals. Health officials and policymakers require robust evidence to formulate effective epidemic control strategies and provide guidelines to contain the ongoing COVID-19 pandemic.

Researchers evaluated Ct-values among distinct groups, namely, a) completely vaccinated and unvaccinated individuals and b) asymptomatic and symptomatic individuals at the time of testing. During the study period, the Delta variant dominantly circulated between two distinct populations. These are Unidos en Salud (UeS) community-based sites in the Mission District of San Francisco and Healthy Yolo Together (HYT) asymptomatic testing through the University of California (UC), Davis.

A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.

The main findings

In this study, a total of 869 samples were analyzed, among which 500 were from the HYT group and 369 from UeS. All the samples belonging to HYT were asymptomatic at the time of sample collection, and researchers revealed that 75% of the COVID-19 positive samples were from unvaccinated individuals.

In the case of the UeS group, COVID-19 positive samples belonged to both symptomatic and asymptomatic individuals. Researchers found that the frequency of vaccine breakthroughs in the UeS group was greater than in the HYT group. As expected, the SARS-CoV-2 Delta variant was the predominant strain in both the study populations.

The authors of this study did not find any statistically significant difference in mean Ct-values between vaccinated and unvaccinated samples. They estimated Ct-values of <15 to >30 in both UeS and HYT samples. Similarly, researchers also did not obtain statistically significant differences in the mean Ct-values between asymptomatic and symptomatic samples.

In this study, researchers also calculated Ct-values among different age groups, genders, and vaccine types. They reported that in all the studied groups, individuals with low Ct-values indicated high viral loads regardless of the vaccine status, age, or gender. This result defies a previous study associated with a large UK-based cohort that reported that the median Ct-value was higher for vaccinated individuals than unvaccinated individuals. Another study conducted in San Francisco revealed that fully vaccinated asymptomatic individuals showed significantly lower viral loads than symptomatic and vaccinated individuals.

The result of the current study is in line with other recently published reports that revealed similar viral loads among vaccinated and unvaccinated individuals in scenarios related to the Delta variant transmission. Similarly, the results of another study conducted in Wisconsin were similar to the reports of the current study, which revealed similar Ct values between vaccinated and unvaccinated cases. In Singapore and Massachusetts, individuals experiencing vaccination breakthrough because of the Delta variant infection showed Ct values similar to unvaccinated individuals.

The authors of this study strongly emphasized that the result obtained in this study is consistent across studies associated with large cohorts using varied assays and locations. The present study found lower Ct-values in asymptomatic and fully vaccinated individuals, consistent with the potential for transmission from breakthrough infections before any emergence of symptoms. However, an earlier study conducted in Singapore indicated that viral loads decreased more rapidly in vaccinated individuals than unvaccinated individuals.

Future research and recommendations

More than 20% of positive COVID-19 recovered patients, and vaccinated individuals showed low Ct-values (<20), a third of which were asymptomatic when tested. This highlights the need for additional studies of the immunological status of such vaccine escapes and how infectious they are. There is a need for more research to determine the immunological status of the vaccination breakthrough cases or the reason for evasion of the immune response.

Further, scientists observed low Ct-values in some children (below 12 years) and those not yet eligible for vaccination. This indicates that children with higher viral loads are infectious and can spread the disease. Thereby, frequent COVID-19 testing for school children is essential for early detection, which would prevent the further spread of the virus.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.


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