The Omicron variant is nearly 3 to 4 times more infectious than the Delta variant among vaccinated persons if a household member becomes infected, suggests a preprint study from Denmark. The booster vaccinated persons are less likely to spread the infection.

The transmission of the Omicron variant in households was investigated, which covered 11,937 households. Researchers addressed the secondary attack rate of Omicron, its immune evasiveness and effectiveness of the booster vaccination in reducing transmission in this study. Data was sourced from Danish registers from 9th December 2021 to 12th December, 2021. Potential secondary cases were followed up for a week to accommodate the time for the test results. The study participants were categorized into three groups based on their vaccination status: Unvaccinated, fully vaccinated and booster vaccinated.

Among the 11,937 households, there were 2225 primary Omicron cases and 9712 primary cases with delta infection. A total of 6397 cases of secondary infections were detected in the week after the first infection in the household.

The secondary attack rates (SAR) with the Omicron and the Delta variants were not very different among unvaccinated potential secondary cases; 29% and 28%, respectively. Among the fully vaccinated persons, SAR was 32% in houses with Omicron infection and 19% in households with the Delta infection. Among the booster-vaccinated persons, the SAR was 25% for Omicron, whereas it was only 11% for delta.

The odds ratio (OR) for infection was 1.17 times higher for unvaccinated persons in households with Omicron infection compared to households with the Delta infection, while the OR for infection was 2.61 times higher for fully vaccinated and 3.66 times higher for those who had taken the booster dose. The higher OR of infection among the vaccinated and the booster vaccinated persons indicate the immune evading nature of the Omicron variant.

This study has shown that among vaccinated persons, the likelihood of spread of the virus was 2.7-3.7 times higher in houses with Omicron infection compared to the delta variant suggesting the more infectious nature of the variant. The unvaccinated persons are more susceptible to the infection than those who are fully vaccinated and booster-vaccinated. The authors further note that since the SAR of Omicron versus Delta were comparable among unvaccinated individuals, the increased transmissibility of Omicron could be due to immune evasion “rather than an inherent increase in the basic transmissibility”.

The transmissibility from unvaccinated persons was higher (OR: 1.41), while the transmissibility was reduced for the booster-vaccinated individuals (OR: 0.72) suggesting that booster vaccination can potentially reduce household transmission of the Omicron variant.

Reference

1. Frederik Plesner Lyngse, et al. SARS-CoV-2 Omicron VOC transmission in Danish households. medRxiv, Posted December 27, 2021. doi: https://doi.org/10.1101/2021.12.27.21268278