In November 2021, the World Health Organization (WHO) classified the rapidly spreading severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) Omicron variant B.1.1.529 as a variant of concern (VOC). This variant has since spread to over 50 countries worldwide.

In a recent study posted on the medRxiv* preprint server, researchers evaluate vaccine effectiveness (VE) against the SARS-CoV-2 Omicron and Delta variants in individuals who have received a complete two-dose vaccine regimen or a third booster dose of the vaccine.

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Study: Vaccine effectiveness against SARS-CoV-2 infection with the Omicron or Delta variants following a two-dose or booster BNT162b2 or mRNA-1273 vaccination series: A Danish cohort study. Image Credit: george martin studio / Shutterstock.com

About the study

In the present study, researchers collected data regarding reverse transcriptase-polymerase chain reaction (RT-PCR)-confirmed SARS-CoV-2 cases in individuals who were fully vaccinated with two-dose or booster BNT162b2 or mRNA-1273 vaccines from Danish health registries between November 20, 2021, and December 12, 2021. In order to differentiate between Delta and Omicron variants, the researchers performed whole-genome sequencing on the RT-PCR-confirmed SARS-CoV-2 samples.

VE was evaluated by a time-to-event analysis by comparing the rate of infection between unvaccinated and vaccinated individuals with two-dose or booster BNT162b2 or mRNA-1273 vaccination series.

Study findings

By the end of the study period on December 12, 2021, 5,767 Omicron cases were identified in Denmark with a median age of 28 years. Initially, VE against the Omicron variant in recipients of the BNT162b2 and mRNA-1273 vaccines were 55.2% and 36.7%, respectively; however, VE declined rapidly thereafter for both vaccines.

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Vaccine effectiveness against SARS-CoV-2 infection with the Delta and Omicron variants, shown separately for the BNT162b2 and mRNA-1273 vaccines. Vertical bars indicate 95% confidence intervals.

For those who had received booster dose 14 to 44 days before the study period, VE rose to an average of 54.6%. The VE from both vaccines showed significantly higher protection against SARS-CoV-2 infection with the Delta variant as compared to the Omicron variant.

The high transmissibility of the Omicron variant further resulted in negative estimates, which indicated differences in the behaviors and exposure patterns of SARS-CoV-2 infection between vaccinated and unvaccinated individuals.

Another study in England found 75.5% VE among BNT162b2 booster-vaccinated individuals. However, the increase in VE after the booster dose also declined rapidly over time, which supports the present study findings.

Conclusions

Overall, the current study showed that protection against the SARS-CoV-2 Omicron variant declines rapidly over time after primary vaccination with BNT162b2 or mRNA-1273 vaccines. However, booster doses can significantly increase protection. Importantly, the researchers observed that VE against SARS-CoV-2 Delta infection is higher in comparison to that against Omicron infection.

“In light of the exponential rise in Omicron cases, there is an essential need for massive rollout of vaccinations and booster vaccinations.”

*Important notice

medRxiv 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

Susha Cheriyedath

Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.