#16,541
While the rate of COVID (primarily Omicron BA.1) infection in the United States, Germany, and the UK have stabilized over the past couple of weeks, in Denmark - where the BA.2 Omicron subvariant is now dominant - infections have continued to climb (see chart above) and are running roughly three times higher.
Last Thursday Denmark's SSI characterized the transmission of BA.2 as being `well over one and a half more contagious than BA.1.', and a week before that the UK Health Security Agency designated BA.2 as a Variant Under Investigation (VUI).
But many of the reports we've seen have been anecdotal. On Friday, in UKHSA: Risk Assessment On Omicron BA.2 Subvariant, which cited limited data, found:
Growth rate
BA.2 has an increased growth rate compared to BA.1 in all regions of England where there are sufficient cases present to make an assessment. Whilst growth rates can be overestimates early in the emergence of a variant, the apparent growth advantage is currently substantial.
Secondary attack rates
Analysis from routine contact tracing data indicated higher secondary attack rates amongst contacts of BA.2 cases in households (13.4%; 95% CI: 10.7%-16.8%) than those for contacts of other Omicron cases (10.3%; 95% CI: 10.1%-10.4%) in the period 27 December 2021 to 11 January 2022. These secondary attack rates are not adjusted for vaccination status and reflect overall growth advantage rather than transmissibility.
Vaccine effectiveness
A preliminary assessment did not find evidence of a difference in vaccine effectiveness against symptomatic disease for BA.2 compared to BA.1. However, numbers included in this study are relatively small and it will be iterated. The University of Oxford has reported preliminary unpublished pseudovirus neutralisation data. In this study, BA.1 and BA.2 pseudoviruses did not differ substantially in neutralisation by sera from vaccinated individuals.
Yesterday, researchers from Denmark's SSI, and Copenhagen University, published the following preprint which compares the transmission dynamics of BA.1 and BA.2.
While no evidence is presented suggesting any increased severity with BA.2, this report does find a much higher SAR (Secondary Attack Rate) in households affected by BA.2.
BA.2 also appears to evade current vaccines (i.e. `Breakthrough infections') even better than BA.1, but - surprisingly - fully vaccinated (and/or boosted) individuals who are infected, appear less capable of transmitting the virus onward to others.
Transmission of SARS-CoV-2 Omicron VOC subvariants BA.1 and BA.2: Evidence from Danish Households
Frederik Plesner Lyngse, Carsten Thure Kirkeby, Matthew Denwood, Lasse Engbo Christiansen, Kåre Mølbak,Camilla Holten Møller, Robert Leo Skov, Tyra Grove Krause, Morten Rasmussen, Raphael Niklaus Sieber, Thor Bech Johannesen, Troels Lillebaek, Jannik Fonager, Anders Fomsgaard, Frederik Trier Møller, Marc Stegger, Maria Overvad,Katja Spiess, Laust Hvas Mortensen
doi: https://doi.org/10.1101/2022.01.28.22270044
Abstract
The Omicron SARS-CoV-2 variant of concern (VOC lineage B.1.1.529), which became dominant in many countries during early 2022, includes several subvariants with strikingly different genetic characteristics. Several countries, including Denmark, have observed the two Omicron subvariants: BA.1 and BA.2.
In Denmark the latter has rapidly replaced the former as the dominant subvariant. Based on nationwide Danish data, we estimate the transmission dynamics of BA.1 and BA.2 following the spread of Omicron VOC within Danish households in late December 2021 and early January 2022.
Among 8,541 primary household cases, of which 2,122 were BA.2, we identified a total of 5,702 secondary infections among 17,945 potential secondary cases during a 1-7 day follow-up period.
The secondary attack rate (SAR) was estimated as 29% and 39% in households infected with Omicron BA.1 and BA.2, respectively. We found BA.2 to be associated with an increased susceptibility of infection for unvaccinated individuals (Odds Ratio (OR) 2.19; 95%-CI 1.58-3.04), fully vaccinated individuals (OR 2.45; 95%-CI 1.77-3.40) and booster-vaccinated individuals (OR 2.99; 95%-CI 2.11-4.24), compared to BA.1.
We also found an increased transmissibility from unvaccinated primary cases in BA.2 households when compared to BA.1 households, with an OR of 2.62 (95%-CI 1.96-3.52). The pattern of increased transmissibility in BA.2 households was not observed for fully vaccinated and booster-vaccinated primary cases, where the OR of transmission was below 1 for BA.2 compared to BA.1.
We conclude that Omicron BA.2 is inherently substantially more transmissible than BA.1, and that it also possesses immune-evasive properties that further reduce the protective effect of vaccination against infection, but do not increase its transmissibility from vaccinated individuals with breakthrough infections.
(SNIP)
The present household study showed a transmission advantage of Omicron BA.2 over BA.1. Although vaccinations, in particular booster vaccinations, did protect against infection, the 2.45 (fully vaccinated) and 2.99 (booster vaccinated) fold higher odds of infection in BA.2 households indicate that BA.2 as a phenotype represents a further step in immune evasion in the Omicron lineage.
However, it is likely that this change came with an evolutionary cost for BA.2. To our surprise, we found a decreased transmissibility of BA.2 relative to BA.1 among fully vaccinated and booster vaccinated, with estimates of 0.60 and 0.62, respectively. Based on such a considerable loss in transmissibility among vaccinated individuals, it is not straightforward to predict the future trajectory of BA.2 relative to BA.1 or other potentially emerging variants.
While the future course of COVID is incredibly difficult to predict, BA.2 - for now, at least - appears to have a competitive advantage over BA.1, and is likely to become increasingly dominant around the globe.
Assuming, of course, that something more `biologically fit' doesn't emerge in the near future.
