Exploring vaccine effectiveness against SARS-CoV-2 Omicron and Delta variants in adolescents

A recent study posted to the medRxiv* preprint server assessed the effectiveness of the BNT162b2 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine against SARS-CoV-2 Delta and Omicron variants in adolescents.

Study: Vaccine effectiveness of BNT162b2 against Omicron and Delta outcomes in adolescents. Image Credit: Tikhonova Yana/Shutterstock
Study: Vaccine effectiveness of BNT162b2 against Omicron and Delta outcomes in adolescents. Image Credit: Tikhonova Yana/Shutterstock

Background

The coronavirus disease 2019 (COVID-19) pandemic has caused widespread morbidity and mortality across all age groups. The development of COVID-19 vaccines played an important role in curbing the transmission and fatal outcomes of the SARS-CoV-2 infections. However, there is limited knowledge on the impact of vaccine administration on COVID-19 outcomes in infected adolescents.

About the study

In the present study, the researchers estimated the effect of two or three doses of COVID-19 vaccines on COVID-19 outcomes in adolescents infected by SARS-CoV-2 Delta or Omicron variants.

The team employed a test-negative study design among adolescents aged between 12 to 17 years. Individuals belonging to this age group who had tested SARS-CoV-2-positive between 22 November 2021 and 6 March 2022 were included in the study.

Data on the laboratory findings, vaccination status, reportable disease, and medical history were linked and assessed for the eligible population at the Institute for Clinical Evaluative Sciences (ICES). Positive SARS-CoV-2 results were further validated using whole-genome sequencing and SARS-CoV-2 spike gene target failure results to determine the causative variant.

The outcomes assessed were vaccine effectiveness against symptomatic SARS-CoV-2 Delta and Omicron infections and COVID-19-associated hospitalizations and deaths. Vaccine efficacy was evaluated over a period of time after the individual received the second or third vaccine dose.

The team also estimated vaccine effectiveness in individuals suffering from symptomatic Omicron infection prior to and after eligibility restrictions were constituted for taking polymerase chain reaction (PCR) tests. Furthermore, the researchers compared the odds of vaccination in infected patients to symptomatic individuals who tested negative for SARS-CoV-2.              

Results

A total of 9,902 SARS-CoV-2 Omicron patients and 502 Delta patients participated in the study. Among the Omicron patients, 91% were vaccinated with two doses while 1.3% were vaccinated with three doses.

Vaccine effectiveness was found to be lower in symptomatic Omicron patients than symptomatic Delta patients. Vaccine effectiveness against Delta and Omicron infections seven to 59 days post-second vaccination was 51% and 97% while 180 days after the second dose it was 29% and 90%, respectively.

The study also found that the two-dose and three-dose vaccine effectiveness against severe COVID-19 outcomes in Omicron patients was 85% and 62% seven or more days after the second and third doses were administered, respectively. Furthermore, the researchers found no significant differences in the vaccine effectiveness before and after the test eligibility restrictions were placed.   

Conclusion

The study findings showed the low vaccine effectiveness, induced by two COVID-19 vaccine doses, necessitated the administration of the third dose in adolescents, which significantly improved protection against severe outcomes in Omicron patients.

 However, the protective effect lasted only during the initial period post-vaccination. The researchers believe that understanding the effect of the third dose in adolescents requires further research.         

*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.

Journal reference:
Bhavana Kunkalikar

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.

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