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SARS-CoV-2 Omicron has an infectious viral load similar to Delta




In a recent study published in medRxiv * Preprint server, researchers compare premutation (VOC) strains and delta VOC strains of concern for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). To do this, researchers have found that unvaccinated individuals and the amount of ribonucleic acid (RNA) and infectious viral load in breakthrough infections of vaccination caused by SARS-CoV-2 delta and Omicron variants. (VL) was compared.

study: Infectious viral load in unvaccinated and vaccinated patients infected with SARS-CoV-2 WT, Delta, and Omicron. Image Credits: DottedYeti /

VL affects secondary infections with SARS-CoV-2. The higher VL is due, at least in part, to the appearance of alpha and delta VOCs.

In addition, it is important to understand VL in post-vaccination breakthrough infections, as some of the population is vaccinated. Since RNA VL is a weak indicator of infectivity, research on the presence of infectious virus by cell culture isolation is important.

About research

In the current study, until 5 days after the onset of symptoms (DPOS), researchers characterized viral load in the upper respiratory tract (URT) of unvaccinated pre-vaccinated coronavirus disease 2019 (COVID-19) participants. I checked as follows. Vaccinated and unvaccinated DeltaVOC-infected COVID-19 subjects. This cohort contained 384 samples, with 118 samples from patients infected with pre-VOC SARS-CoV-2 and 248 samples from patients infected with DeltaVOC.

A total of 121 DeltaVOC-infected patients were vaccinated twice prior to infection, while 127 were unvaccinated. The study also included 18 vaccinated people who were recently infected with Omicron. Using quantitative real-time PCR (qRT-PCR) and focal formation assay, researchers measured the genomic copy and infectious virus titers of SARS-CoV-2 positive nasopharyngeal swabs.

Researchers first investigated whether RNA genomic copy was a good predictor of infectivity. Viral shedding.. Next, we determined whether the amount of infectious virus in the patient sample was related to the patient’s age and gender.

During the first 5 days of symptoms, researchers compared pre-VOC and Delta VOC sample genome copies from unvaccinated patients with infectious virus levels. They further evaluated the genomic copy and infectious viral load of non-vaccinated and vaccinated patients infected with DeltaVOC throughout the first 5 days after infection to determine if immunity affected viral shedding.

Researchers also investigated infectious viral shedding in vaccinated patients infected after the emergence of a new VOC omicron strain.of Nasopharyngeal swab Samples, RNA, and infectious viral load levels from 18 Omicron and 17 Delta-infected patients were also compared.

Investigation result

The results showed that unvaccinated Delta-infected individuals had higher levels of infectious virus than pre-VOC-infected patients, and vaccinated patients had much lower levels of infectious virus. There was no difference in infectious viral load between the breakthrough cases of Delta and Omicron.

RNA genomic copies of URT swab samples were slightly associated with infectious viral shedding. In fact, researchers found more RNA genomic copies in unvaccinated individual swabs infected with pre-VOC SARS-CoV-2 strains than in Delta VOCs in the first five DPOS. At the same time, the infectious virus titer of DeltaVOC was high.

During the first five days after the onset of symptoms, researchers were able to show that vaccinated Delta VOC patients had reduced infectious viral load rather than RNA genomic copy. In addition, about 50% of pre-VOC SARS-CoV-2 strain infections occur during this time frame, which can significantly contribute to lower viral load and lower secondary incidence. is showing.

According to the data, the higher infectivity of the SARS-CoV-2 omicron variant is not associated with the elimination of large numbers of infectious virus particles in vaccinated patients with comparable RNA viral load and infectious viral shedding. ..

Quantitative infectious viral load and successful overall virus isolation.

Quantitative infectious viral load and overall successful virus isolation..


Only samples with a cycle threshold (Ct) value less than 27 were included and were captured within the first 5 DPOS, but not after that. Since patients with low viral load were not included, the absolute number of RNA copies is biased towards high viral load.

In addition, researchers focused on infectious viral shedding. For this purpose, SARS-CoV-2 cultures are unlikely to succeed from samples with higher Ct values, demonstrating that the majority of secondary infections occur before 5 DPOS. However, this needs to be formally evaluated in the case of Omicron.

Researchers also lacked access to samples from unvaccinated people infected with Omicron due to the recent emergence of this strain. Finally, the researchers showed that almost all patients in this study were vaccinated with the messenger ribonucleic acid (mRNA) vaccine. Neutralizing antibody Mucosal antibody with low titer. As a result, the findings do not apply to other vaccines, especially those used primarily in low- and middle-income countries.

RNA virus levels and infectious virus titers of non-vaccinated and vaccinated Delta-infected (A) Genome copies of vaccinated and non-vaccinated Delta-infected patients (left panel) and infectious virus (right panel) ). Error bars indicate average ± SD.  The t-test was used to determine the difference between the means.  *** p = 0 ・ 0005; **** p <0・0001。 異なるdposでワクチン接種および非ワクチン接種のDelta感染患者について測定されたゲノムコピー(B)および感染性ウイルス量(C)。 実線は、(局所的に推定された散布図の平滑化)LOESS法を使用して計算された近似曲線を表しています。

RNA virus levels and infectious virus titers of non-vaccinated and vaccinated Delta-infected (A) Genome copies of vaccinated and non-vaccinated Delta-infected patients (left panel) and infectious virus (right panel) ). Error bars indicate average ± SD. The t-test was used to determine the difference between the means. *** p = 0 ・ 0005; **** p <0 ・ 0001. Genome copy (B) and infectious viral load (C) measured for vaccinated and non-vaccinated Delta-infected patients at different dpos. The solid line represents the fitted curve calculated using the LOESS method (locally estimated scatter plot smoothing).


In summary, research results show that Delta VOCs are highly infectious, but are much less infectious and have faster viral clearance in vaccinated individuals. In addition, findings show that Omicron and Delta VOCs have the same infectious viral load. In addition to overall viral isolation performance, viral load measurements can also provide excellent insight into viral shedding dynamics in acute SARS-CoV-2 infections.

“Higher RNA genomic copies were observed in SARS-CoV-2 before VOC compared to Delta, but significantly higher IVT was observed in Delta-infected individuals.”

*Important Notices

medRxiv Publish preliminary scientific reports that should not be considered definitive as they have not been peer-reviewed, guide clinical practice / health-related behaviors, and should not be treated as established information.





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