BA.2 Blitzes Through Households, Even More Than Original Omicron

Omicron subvariant BA.2 was associated with higher susceptibility of infection among households, regardless of vaccination status, than the original BA.1 variant, Danish researchers found.

Compared with the original variant, BA.2 was linked with more than two times higher odds of infection among unvaccinated individuals (OR 2.19, 95% CI 1.58-3.04), vaccinated individuals (OR 2.45, 95% CI 1.77-3.40), and boosted individuals (OR 2.99, 95% CI 2.11-4.24), reported Frederik Plesner Lyngse, MD, of University of Copenhagen, and colleagues.

Indeed, unvaccinated primary cases in BA.2 households were associated with more than two times higher odds of transmission (OR 2.62, 95% CI 1.96-3.52) compared to BA.1 households, the authors wrote in a preprint on medRxiv.

Not surprisingly, the authors found lower transmissibility in both BA.1 and BA.2 households when the primary case was vaccinated with or without a booster.

While there was significantly increased transmissibility associated with BA.2 households versus BA.1 when the primary case was unvaccinated, it was non-significant when the primary case was vaccinated and non-boosted or boosted, according to the authors.

BA.2 has recently garnered media attention due to its higher transmissibility. Lyngse’s group noted that the two variants “differ by approximately 40 mutations.”

Lyngse’s group said the increasing number of BA.2 cases led them to explore two issues through their study: “Is there a difference in the household transmission patterns between … BA.1 and BA.2; and … if there is a difference, is it due to a difference in susceptibility, transmissibility, or both, and could this indicate a difference in immune evasiveness between the subvariants?”

They used Danish registry data that included households with two to six members, from Dec. 20, 2021 to Jan, 11, 2022, with a 7-day follow-up period until Jan. 18, 2022.

Primary cases were identified via RT-PCR, followed by whole-genome sequencing to determine if the variant was BA.1 or BA.2. Households were identified as BA.1 or BA.2 depending on the sequencing of the primary case.

Overall, there were 2,122 BA.2 households, with a total of 4,587 potential secondary cases and, of those, 1,792 tested positive for an estimated secondary attack rate of 39%. There were 6,419 BA.1 households, with a total of 13,358 potential secondary cases and, of those, 3,910 tested positive for an estimated secondary attack rate of 29%.

Notably, a 14-day follow-up found similar patterns, with a secondary attack rate of 42% for BA.2 versus 36% for BA.1.

“A potential mechanism for the higher transmissibility of unvaccinated individuals infected with BA.2 compared to BA.1 could be a higher viral load,” the authors wrote, adding because no such difference was found in vaccinated non-boosted or boosted individuals, this “could be a result of a lower viral load in vaccinated individuals with a breakthrough infection.”

Limitations to the data include that it was collected over Christmas and New Year’s, when families held communal gatherings, so some co-primary cases were most likely misclassified as secondary cases, the authors noted.