COVID-19 Scan for Sep 29, 2022

COVID Omicron variant shown to spread better in households than DeltaThe SARS-CoV-2 Omicron variant is much more transmissible among household members than Delta, with an estimated secondary attack rate (SAR) from thrice-vaccinated index patients of 46%, compared with 11% for Delta, finds a study published today in Nature Communications.The SAR is the percentage of contacts who become infected by an index (initial) patient.Norwegian Institute of Public Health researchers used contact tracing data to track the 10-day SAR of Omicron and Delta among 1,122 index patients infected with Omicron (59%) or Delta (41%) and 2,169 household contacts (60% for Omicron primary cases and 41% for Delta) from December 2021 to January 2022.

The data represented 8 of Norway's 11 counties.The estimated overall Omicron SAR was 51%, compared with 36% for Delta. The SAR among thrice-vaccinated index patients was 46%, versus 11% for Delta (relative risk [RR], 1.41).

Although the average SAR was lower for both variants among contacts vaccinated with three doses, transmission was higher with Omicron than Delta among all vaccination groups except one-dose recipients.Both twice- and thrice-vaccinated index patients were at comparable risk as unvaccinated index patients of spreading Omicron to adult household contacts (RR, 1.04 and 0.99, respectively).

In contrast, Delta index patients who received three vaccine doses were at an 82% lower risk of spreading Delta (RR, 0.18).There were no significant differences in risk of infection by age-group, sex, or time since the last vaccine dose in twice-vaccinated contacts.Vaccine effectiveness (VE) for thrice-vaccinated adult household contacts was lower for Omicron than Delta (45% vs 65%) and higher than