Does Omicron spread faster than Delta in Vaccinated People?


A Danish study of nearly 12,000 households has discovered that Omicron spreads faster than Delta among those who are fully vaccinated, and even higher between those who have received booster shots, demonstrating strong evidence of the variant’s immune evasiveness.

Unvaccinated Persons Spread the Virus More Easily than Fully Vaccinated Persons

The Omicron variant was found to evade the immunity of vaccinated individuals at a much faster pace compared to Delta, and at a higher rate than the unvaccinated, according to the study conducted by researchers at the University of Copenhagen, Statistics Denmark, and Statens Serum Institut.

“Comparing households infected with the Omicron to Delta VOC, we found an 1.17 times higher SAR 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 VOC,” said the preprint of the study. SAR refers to secondary attack rate.

However, the study also discovered that unvaccinated individuals spread the virus more easily than those who are fully vaccinated, while there was reduced transmission between people who received booster shots.

There were altogether 11,937 households involved in the study, out of which 2,225 already had Omicron. After one to 7 days, the team followed up with the households and found 6,397 secondary infections. The SAR was found to be 31 percent with the Omicron, and 21 percent with the Delta variant.

Omicron has spread to 90 countries worldwide and has become the dominant variant in the United States. However, the strain has proven to cause mild symptoms compared to earlier variants like Delta, and results in fewer hospitalizations.

“The Omicron VOC (variant of concern) has been reported to be three to six times as infectious as previous variants, with a short doubling time, including early estimates from countries with a high vaccination coverage indicating doubling times of 1.8 days (UK), 1.6 days (Denmark), 2.4 days (Scotland) and 2.0 days (United States),” from the study.

The researchers also suggested considering alternate methods for combating the infection. “Our data indicate that the non-pharmaceutical interventions that were used to control the previous variants of SARS-CoV-2 are also likely to be effective against the Omicron VOC.”

Seventy-eight percent of Danish citizens are fully vaccinated with two doses, while almost half have received a booster shot. More than 80 percent have received Pfizer-BioNTech’s vaccine.

The effectiveness of vaccines is reduced to around 40 percent for symptoms and 80 percent for severe disease with the Omicron variant. Numbers were better with booster shots as the effectiveness was reduced only to 86 percent for symptoms and 98 percent respectively for those suffering from severe infection.

The vaccination effectiveness with Pfizer-Biotech for preventing infection with the Omicron variant is only 35 percent, the study found. “The advantage of the Omicron VOC seems to be a combination of high transmissibility and increased immune evading abilities.”

“We therefore suggest that adapted or improved vaccines may be necessary to mitigate the spread of the Omicron VOC.”

The Danish team added that more studies are required to understand the latest COVID-19 variant, as they concluded, “Our results confirm that booster vaccination has the potential to reduce Omicron VOC transmission in households, although vaccination as a strategy for epidemic control is increasingly challenged by the immune evasiveness of the Omicron VOC.”

A preprint of the study was published on Dec. 27, and it has not yet been peer-reviewed.