| dc.description.abstract | Data collected in the Global COVID-19 Trends and Impact Surveys
(UMD Global CTIS), and data on variants sequencing from GISAID,
are used to evaluate the impact of the Omicron variant (in South
Africa and other countries) on the prevalence of COVID-19 among
unvaccinated and vaccinated population, in general and discrim-
inating by the number of doses. In South Africa, we observe that
the prevalence of COVID-19 in December (with strong presence
of Omicron) among the unvaccinated population is comparable to
the prevalence during the previous wave (in August-September), in
which Delta was the variant with the largest presence. However,
among vaccinated, the prevalence of COVID-19 in December is
much higher than in the previous wave. In fact, a significant re-
duction of the vaccine efficacy is observed from August-September
to December. For instance, the efficacy drops from 0.81 to 0.30 for
those vaccinated with 2 doses, and from 0.51 to 0.09 for those vacci-
nated with one dose. The study is then extended to other countries
in which Omicron has been detected, comparing the situation in
October (before Omicron) with that of December. While the re-
duction measured is smaller than in South Africa, we still found,
for instance, an average drop in vaccine efficacy from 0.53 to 0.45
among those vaccinated with two doses. Moreover, we found a
significant negative (Pearson) correlation of around −0.6 between
the measured prevalence of Omicron and the vaccine efficacy. | es |
