Secondary attack rates for Omicron and Delta variants

Editor's Note

This study from Norway, a country with a 90% primary vaccination rate, finds that a secondary attack rate of COVID-19 was higher when the individual's primary case was the Omicron variant.

All Norwegian residents were tracked from December 1, 2021, to January 8, 2022, the period when Omicron increased from less than 5% to more than 85% of all variant isolates in Norway.

Of 31,220 households with 1 COVID-19 index case:

• 11,643 individuals belonged to a household in which the variant of the index case was Omicron.
• 41,015 individuals belonged to a household in which the variant of the index case was Delta.
• 28,299 individuals belonged to a household in which the variant of the index case was nonclassified.

The secondary attack rate of COVID-19 was:

• 25.1% when the variant of the index case was Omicron.
• 19.4% when the variant of the index case was Delta.
• 17.9% when the variant of the index case was nonclassified.

Adjusted logistic regression analysis finds odds ratios of secondary attacks were higher for men, unvaccinated individuals, and those older than 30 years.