SARS-CoV-2 Omicron VOC Transmission in Danish Households

Abstract

The Omicron variant of concern (VOC) is a rapidly spreading variant of SARS-CoV-2 that is likely to overtake the previously dominant Delta VOC in many countries by the end of 2021. We estimated the transmission dynamics following the spread of Omicron VOC within Danish households during December 2021. We used data from Danish registers to estimate the household secondary attack rate (SAR). Among 11,937 households (2,225 with the Omicron VOC), we identified 6,397 secondary infections during a 1-7 day follow-up period. The SAR was 31\% and 21\% in households with the Omicron and Delta VOC, respectively. We found an increased transmission for unvaccinated individuals, and a reduced transmission for booster-vaccinated individuals, compared to fully vaccinated individuals. Comparing households infected with the Omicron to Delta VOC, we found an 1.17 (95\%-CI: 0.99-1.38) times higher SAR for unvaccinated, 2.61 times (95\%-CI: 2.34-2.90) higher for fully vaccinated and 3.66 (95\%-CI: 2.65-5.05) times higher for booster-vaccinated individuals, demonstrating strong evidence of immune evasiveness of the Omicron VOC. Our findings confirm that the rapid spread of the Omicron VOC primarily can be ascribed to the immune evasiveness rather than an inherent increase in the basic transmissibility.

Interplay Between Risk Perception, Behavior, and COVID-19 Spread

“…We furthermore include that those people having received a booster vaccine maintain some protection against infection with Omicron, which, however, also wanes. These assumptions are consistent with a large Danish cohort of households, where the secondary attack rate among unvaccinated was slightly higher for Delta infections than for Omicron [47], and with extensive experimental and observational studies [32,38,62,63]. Despite the approximation we did for the transition to the Omicron variant, the mid-and long-term dynamics of the Omicron VOC should be reflected well.…”

“…At the very least, modeling these dynamics would require having separate compartments for all the disease states of all circulating variants, disproportionally increasing the complexity of our model. In our approach, we take advantage of the extensive immune escape of the Omicron VOC to natural and vaccineelicited neutralization [29,31,32,45,47], and assume that the replacement of Delta VOC occures very quickly (i.e., basically instantaneously) in mid-January. This simplification is not too distant from reality; replacement of Delta and other predominant sublineages for Omicron took only a few weeks in several countries [61].…”

“…This variant is rapidly replacing the Delta VOC, thus posing an imminent risk. Although there is substantial uncertainty about its epidemiological features, preliminary evidence shows: Compared to the Delta VOC, Omicron exhibits 1) an increased risk of reinfection or break-through infection [29][30][31], 2) a substantial reduction in antibody neutralization [32][33][34][35][36][37][38], 3) a reduction in vaccine effectiveness against infection [31,37,[39][40][41][42][43][44], and 4) faster spread [30,31,45,46] mainly due to immune escape [47].…”

What do we know about covid vaccines and preventing transmission?

“…Few studies have looked at the omicron variant, although a report published in January 2022 by the European Centre for Disease Prevention and Control cited a small Danish household study:6 “People who have completed the primary series of vaccination experienced secondary attack rates (SARs) of 32% in households with omicron and 19% in households with delta. For people who received a booster, omicron was associated with a SAR of 25%, while the corresponding estimate for delta was only 11%.…”

Virus neutralization of SARS-CoV-2 Omicron variant BA.2 in those vaccinated with three doses of BNT162b2 or CoronaVac vaccines.

“…Three subvariants of Omicron were noted at the time of its first recognition, designated BA.1, BA.2 and BA.3, but the subvariant that initially spread world-wide was BA.1 3 . It had had 37 amino acid changes in the virus spike protein compared to the wild type virus, had increased transmissibility relative to all previous virus variants and rapidly became the dominant SARS-CoV-2 world-wide 4 . It appears to have altered tropism for the human respiratory tract with less efficient replication in the lung parenchyma associated with reduced disease severity 5,6 .…”