Study findings suggest spread of Omicron can be ascribed to immune evasiveness rather than an increase in transmissibility

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The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron (B.1.1.529) variant of concern (VOC) is a rapidly progressing strain that is expected to supersede the formerly widespread Delta VOC in many nations. In South Africa, the Omicron VOC has surpassed the Delta VOC and easily spread to at least 28 countries in Europe, Asia, the Middle East, and South America.

Study: SARS-CoV-2 Omicron VOC Transmission in Danish Households. Image Credit: CGN089 /


The Omicron variant has been estimated to be three to six times more infectious than previous variants with a short doubling time. In fact, early projections suggest a doubling duration of 1.8 days (United Kingdom), 1.6 days (Denmark), 2.4 days (Scotland), and 2.0 days (Scotland and the United States). Furthermore, dissemination of the Omicron VOC has also been reported among individuals who have been fully vaccinated against SARS-CoV-2 and in those with a history of coronavirus disease 2019 (COVID-19).

Data on the transmission of the Omicron VOC is inadequate; however, this information is essential for the effective global control of this variant. Furthermore, it is critical to determine if the growth advantage of this variant is due to immune evasion or the increased inherent transmissibility for this variant.

The ability of the Omicron VOC to evade immunity induced by vaccines is a source of concern worldwide. One preliminary meta-analysis of neutralization studies found that vaccine effectiveness curtailed to around 40% against symptoms and 80% against severe disease; however, booster vaccinations appear to have efficacies of 86% and 98%, respectively.

About the study

A recent study published on the medRxiv* preprint server sought to investigate the household transmission of the Omicron VOC by examining the secondary attack rate and increased immune evasiveness of both the Omicron and Delta variants. This study also assessed the efficacy of booster immunization in reducing virus transmission rates.

The present study evaluated the transmission dynamics that would occur if Omicron VOC spread amongst Danish households. Here, household secondary attack rate (SAR) was estimated using data from Danish registers.

Study participants were separated into three categories according to their vaccination status, including unvaccinated, fully vaccinated, and booster-vaccinated. Participants who were partially vaccinated were considered unvaccinated in this study.

Study findings

The data indicated that the SAR was 31% in houses that encountered the Omicron VOC and 21% in households exposed to the Delta VOC. Across all age groups, the estimated SAR for the Omicron VOC was greater than that for the Delta VOC.

Unvaccinated prospective secondary cases had similar attack rates in households with the Omicron and Delta VOCs at 29% and 28%, respectively. However, fully vaccinated individuals had SAR of 32% in households with the Omicron and 19% in households with the Delta VOC. Meanwhile, Omicron rendered a SAR of 25% in booster-vaccinated individuals, whereas Delta correlated to a SAR of only 11% in this cohort.

Seven days after the primary episode, which was considered a positive test result in this study, the cumulative chance of probable secondary cases rose from 33-41% to roughly 87-89%. The chances of secondary infection were higher in cases where the primary case, which was the first individual within a household to test positive, was with the Delta VOC rather than with the Omicron VOC.

On day seven after the primary case tested positive, the likelihood of potential secondary cases rose increased by two-fold. When the primary case was infected with the Delta VOC or the Omicron VOC, the risk of probable secondary cases testing positive increased from 3% to 4% on day one. Comparatively, the probability increased to 21% and 31% on day seven when the primary case was infected with Delta VOC and Omicron VOC, respectively.

The study examined the distribution of cycle threshold (Ct) values in primary cases with the Omicron and Delta variants. The findings indicated that the difference in the transmission of the Omicron and Delta variants is not related to changes in viral load in the primary case. Notably, the distribution of time since the last vaccination/booster/infection among positive secondary cases was nearly identical across the two variations.

The current study also revealed an intra-household association of 98% and 97.7% between primary case and secondary case variants in households with primary cases of the Omicron VOC and the Delta VOC, respectively. This confirmed the hypothesis that subsequent infections within the same home are likely to be caused by the same variant.

Across all age groups, the SAR for the Omicron VOC was found to be higher than for the Delta VOC. Of note, the SAR for those aged 70 years and above was 32% and 18% for the Omicron and Delta variants, respectively. This has ramifications for care facilities, as it emphasizes the need for increased transmission protection to prevent the spread of the Omicron VOC.


To summarize, the study findings demonstrate that unvaccinated persons were more susceptible to infection by the SARS-CoV-2 Omicron variant, whereas booster-vaccinated individuals were less susceptible as compared to fully vaccinated individuals in Delta VOC-infected households. Furthermore, a lower susceptibility for booster-vaccinated individuals was detected among those living in families infected with the Omicron VOC. In addition, there was enhanced transmissibility from unvaccinated individuals and decreased transmissibility from booster-vaccinated individuals.

Moreover, unvaccinated, fully vaccinated, and booster-vaccinated people had a higher transmission rate in households infected with the Omicron VOC as compared to the Delta variant. When comparing the Omicron variant to the Delta variant, unvaccinated individuals had a 1.17 times higher SAR, whereas fully vaccinated and booster-vaccinated individuals had a 2.61 and 3.66 times higher SAR, respectfully. These observations provide strong evidence of immune evasion of the Omicron VOC.

The findings from the current study imply that booster immunization can lower Omicron VOC transmission in households. However, vaccination as an epidemic control strategy may falter due to the immunological evasiveness of the SARS-CoV-2 Omicron variant.

*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:
Nidhi Saha

Written by

Nidhi Saha

I am a medical content writer and editor. My interests lie in public health awareness and medical communication. I have worked as a clinical dentist and as a consultant research writer in an Indian medical publishing house. It is my constant endeavor is to update knowledge on newer treatment modalities relating to various medical fields. I have also aided in proofreading and publication of manuscripts in accredited medical journals. I like to sketch, read and listen to music in my leisure time.


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    Since "fully vaccinated individuals had SAR of 32% in households with the Omicron" that is greated tthan the average 31% how do we conclude that "there was enhanced transmissibility from unvaccinated individuals"?

    • Odd
      Ian Dunn Ian Dunn Australia says:

      If one or two percent leads to the statement of effectively impugning the status of the unvaccinated it tells us all we need to know about the narrative that must be adhered to. Sad what science has become.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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