Impact of Omicron on COVID-19 prevalence among unvaccinated and vaccinated in Omicron-dominated countries

A team of international scientists has recently explored the impact of the omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on the prevalence of coronavirus disease 2019 (COVID-19) in vaccinated and unvaccinated individuals. The findings reveal that COVID-19 prevalence is significantly higher in vaccinated individuals during the omicron-dominated wave compared to that during the delta-dominated wave. However, in unvaccinated individuals, both delta and omicron variants have a similar impact on COVID-19 prevalence. The study is currently available on the Research Square* preprint server.

Background

Since its emergence in November 2021 in South Africa, the omicron variant of SARS-CoV-2 has caused a sharp rise in new infections worldwide. Because of a heavily mutated spike protein, the variant is expected to have a significantly higher ability to evade pre-existing host immunity induced by natural infection or vaccination. A sharp induction in breakthrough infections in omicron-dominated countries signifies improved immune fitness of the variant.

In 2020, researchers from the University of Maryland, USA, together with Facebook, conducted a large-scale survey (UMD Global CTIS) to prepare a database of self-reported SARS-CoV-2 infections, symptoms, testing, and COVID-19 vaccination status. Afterward, the researchers have used the survey data to assess the differences in vaccine efficacy between delta- and omicron-dominated waves. They have observed a sharp reduction in vaccine efficacy against the omicron variant.

The study

In the current study, scientists have used the UMD Global CTIS survey data to evaluate the prevalence of COVID-19 and vaccine efficacy among unvaccinated, partially vaccinated (single dose), and fully vaccinated (double dose) individuals in South Africa and other omicron-dominated countries.

The UMD Global CTIS survey collects more than 100,000 self-reported data daily worldwide. This survey uses self-reported symptoms as a proxy for active COVID-19 cases. However, a symptom-based proxy can sometimes overestimate the number of cases as similar symptoms might be shared by COVID-19 and other health conditions.

The scientists have re-analyzed the dataset using a machine-learning-based classifier to derive COVID-19 active cases to overcome this uncertainty. They have defined COVID-19 prevalence as the ratio of positive cases to the total number of responses.  

COVID-19 prevalence and vaccine efficacy in South Africa

The prevalence of COVID-19 in South Africa was determined from June to December 2021. The analysis revealed that COVID-19 prevalence during the delta-dominated wave (August – September) in unvaccinated individuals is comparable to that during the omicron-dominate wave (December). In contrast, a significantly higher prevalence was observed among partially and fully vaccinated individuals during the omicron wave. Compared to partial vaccination, full vaccination provided a higher level of protection.

Regarding vaccine efficacy against infection, a clear reduction was observed during the omicron wave compared to that during the delta wave.

COVID-19 prevalence and vaccine efficacy in omicron-dominated countries

Similar to South Africa, COVID-19 prevalence and vaccine efficacy were determined for October (pre-omicron period) and December in a total of 24 omicron-dominated countries.

The analysis revealed a considerable drop in vaccine efficacy in fully vaccinated individuals during the omicron-dominated wave. However, the amplitude of reduction was smaller than that observed in South Africa. A significant negative correlation was also observed between vaccine efficacy and COVID-19 prevalence.

Study significance

The study demonstrates an increased prevalence of COVID-19 cases during the omicron-dominated wave in South Africa and other countries. The induction in prevalence is accompanied by a reduction in vaccine efficacy against SARS-CoV-2 infection. However, the study does not provide information on vaccine efficacy against severe COVID-19, hospitalization, and mortality.

*Important notice

Research Square publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Dr. Sanchari Sinha Dutta

Dr. Sanchari Sinha Dutta is a science communicator who believes in spreading the power of science in every corner of the world. She has a Bachelor of Science (B.Sc.) degree and a Master's of Science (M.Sc.) in biology and human physiology. Following her Master's degree, Sanchari went on to study a Ph.D. in human physiology. She has authored more than 10 original research articles, all of which have been published in world renowned international journals.