New study shows Omicron can evade immunity better than Delta

The Omicron variant of Covid-19, rapidly spreading across the world, has the ability to escape immunity much better than the previous Delta strain, according to a study of Danish households.

The SARS-CoV-2 variant B.1.1.529, which is referred to as the Omicron variant of concern (VOC), has overtaken the Delta in South Africa as well as the US, the UK, and several other countries. It has also spread rapidly to more than 100 countries.A

The study, which is yet to be peer reviewed, led by researchers from the University of Copenhagen and Danish Ministry of Health and others, estimated the transmission dynamics following the spread of Omicron within Danish households during December 2021.

Among 11,937 households (2,225 cases with the Omicron and 9,712 cases with the Delta), the team identified 6,397 secondary infections during a 1-7 day follow-up period. The secondary attack rate was 31 per cent and 21 per cent in households with the Omicron and Delta, respectively.

The study compared households infected with the Omicron to Delta and found an 1.17 times higher secondary attack rate for unvaccinated, 2.61 times higher for fully vaccinated and 3.66 times higher for booster-vaccinated individuals, demonstrating strong evidence of immune evasiveness of the Omicron.

"We found an increased transmission for unvaccinated individuals, and a reduced transmission for booster-vaccinated individuals, compared to fully vaccinated individuals," said Frederik Plesner Lyngse, from the varsity's Department of Economics and Center for Economic Behaviour and Inequality.

Moreover, the study showed that the Omicron is generally 2.7-3.7 times more infectious than the Delta among vaccinated individuals, with a short doubling time.

This is in line with early estimates from countries with a high vaccination coverage indicating doubling times of 1.8 days (the UK), 1.6 days (Denmark), 2.4 days (Scotland) and 2.0 days (the US), the researchers said.

Transmission of the Omicron has been high among individuals being fully vaccinated against SARS-CoV-2 infection as well as among individuals with a history of Covid-19 infection.Studies have indicated that the vaccine effectiveness is reduced to around 40 per cent against symptoms and to 80 per cent against severe disease, but that the effect for booster vaccinations is at 86 per cent and 98 per cent, respectively.

The study also showed that booster-vaccinated individuals generally had a reduced transmissibility and that unvaccinated individuals had a higher transmissibility, compared to fully vaccinated individuals.

Further, the researchers observed no significant difference between the secondary attack rate of Omicron versus Delta among unvaccinated individuals.

Thus, "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", Lyngse said.

Lyngse also noted that the current vaccines are unlikely to mitigate the spread of the Omicron to the extent that has been achieved for previous variants in the long term.

"We therefore suggest that adapted or improved vaccines may be necessary to mitigate the spread of the Omicron VOC," he said.