JMIR Publications

ABSTRACT

Background:

The Omicron variant of SARS-CoV-2 is more transmissible than prior variants of concern (VOCs). It set 2-year record highs in the number of daily new transmissions and displaced Delta as the most transmissible VOC. Omicron has a significant growth advantage over prior VOCs, with a doubling period of 1.5–3 days and a magnitude 1.5 to 2–fold higher than prior outbreaks at its apex. Early data on the spread of Omicron were captured in countries with relatively low case counts, so it was unclear how the arrival of Omicron would impact the trajectory of the pandemic in countries already experiencing high levels of community transmission of Delta.

Objective:

The objective of this study is to quantify and explain the impact of Omicron on pandemic trajectories and how they differ between countries who were or were not in a SARS-CoV-2 outbreak at the time Omicron occurred.

Methods:

We used SARS-CoV-2 surveillance and genetic sequence data to classify countries into two groups: those that were in a Delta outbreak (defined by at least ten novel daily transmissions per 100,000 population) when Omicron was first sequenced in the country, and those that were not. We used trend analysis, survival curves, and dynamic panel regression models to compare outbreaks in the two groups. We summarized the outbreaks in terms of their peak rate of SARS-CoV-2 infections and the duration of time the outbreaks took to reach the peak rate.

Results:

Countries that were already in an outbreak with predominantly Delta lineages when Omicron arrived took longer to reach their peak rate and saw greater than a two-fold increase (2.04) in the average apex of the Omicron outbreak compared to countries that were not yet in outbreak.

Conclusions:

These results suggest that high community transmission of Delta at the time of the first detection of Omicron was not protective, but rather preluded larger outbreaks in those countries. Outbreak status may reflect a generally susceptible population, due to overlapping factors including climate, policy, and individual behavior. In the absence of strong mitigation measures, arrival of a new, more transmissible variant in these countries is therefore more likely to lead to larger outbreaks. Alternately, countries with enhanced surveillance programs and incentives may be more likely to both exist in an outbreak status and detect more cases during an outbreak, resulting in a spurious relationship. Either way, these data argue against herd immunity mitigating future outbreaks with variants that have undergone significant antigenic shifts.