Age-specific susceptibility during the Omicron driven, Delta driven, and pre-Delta waves in South Korea

In a recent study posted to the Research Square* preprint server, researchers assessed the relative age-varying susceptibility to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron strain infections compared to the SARS-CoV-2 Delta strain and pre-Delta strains in South Korea.

Background

Omicron (B.1.1.529) is reported to have higher transmissibility compared to previous SARS-CoV-2 strains, particularly in children, and Omicron pediatric cases are on the rise, which could be due to the prioritization of elders for coronavirus disease 2019 (COVID-19) vaccinations and higher contact among school-going children compared to adults.

However, confirmatory data on the relative susceptibility to Omicron infections among individuals of different ages are lacking. The authors of the present study previously described a model for estimation of the age-varying susceptibility for Delta infections.

About the study

In the present study, researchers updated their previous model by assessing the age-varying susceptibility to Omicron infections relative to Delta and pre-Delta infections in South Korea.

Age-structured daily COVID-19 incidence rates and vaccine uptake rates were obtained from the Ministry of Health and Welfare of South Korea. The vaccination data (of manufacturers and doses) were provided by the Korea Disease Control and Prevention Agency. Age-varying population data were obtained from Statistics Korea.

The period between 1 January and 31 January 2022 was considered the Omicron predominance period (fifth wave), and between 27 June and 21 August, 2021 was considered the Delta dominance period (fourth wave). The pre-Delta strains’ dominance period (third wave) was between 15 October and 22 December 2020.

Age-stratified mathematical compartmental models and the Bayesian statistical inference method were used for the analysis with five-year age bands, and data were adjusted for contact patterns, the status of vaccination, and vaccine efficacy diminution. In addition, a sensitivity analysis was performed by varying the number of contacts in schools and increasing the proportion of asymptomatic infections to 50% across all ages.

The model compartments were stratified by status of infection (i.e., exposure [E], susceptible [S], infectious and asymptomatic [Iasym], infectious and pre-symptomatic [Ipresym], infectious and symptomatic [Isym], and quarantined [Q]) age band, and the transition time to the next infection state.

Findings of an adopted prospective household cohort study that reported age-stratified asymptomatic case proportions (i.e., 52%, 50%, 45%, and 12% among individuals aged below four years, five to 11 years, 12 to 17 years, and ≥18 years, respectively) were considered as baseline data for the proportion of asymptomatic cases. Likewise, for vaccine efficacy, the upper and lower limits of the 95% confidence interval (CI) of sensitivity analyses reported in a previous study were considered.

Results and discussion

The percentages of SARS-CoV-2 infection cases among individuals aged ≤19 years were 11.0%, 16.7, and 28.6% during the third (pre-Delta), fourth (Delta), and fifth (Omicron) waves, respectively. The corresponding proportions of COVID-19 cases in individuals aged ≥60 years were 28.5%, 10.2%, and 10.0%, respectively. On data adjustments (age was skewed older), the corresponding proportions of COVID-19 cases among individuals aged ≤19 years were 13.3%, 23.4%, and 36.9%, respectively.

Age-dependent increases were similar for the pre-Delta (third) and Delta (fourth) COVID-19 waves whereas, for the Omicron (fifth) wave, an inverted bell-shaped curve was observed, indicating substantial differences in Omicron infection susceptibility compared to that for pre-Delta and Delta infections among younger individuals.

The most prominent difference was observed for individuals aged 10 to 15 years which was 5.28-fold higher Omicron strain/pre-Delta strain infection susceptibility and two-fold higher Omicron susceptibility among individuals aged ≥50 years. The least differences were observed for individuals aged ≥75 years. Likewise, the increase in Omicron/Delta susceptibility was greatest for 15 to 19-year-old individuals (3.2-fold higher) and least for individuals aged ≥75 years (0.9-fold higher).

Varying the vaccine effectiveness, contact patterns, and asymptomatic case proportions did not alter the increase in Omicron strain/pre-Omicron strain infection susceptibility, with higher values for individuals aged below 19 years, five-fold higher than pre-Delta strain infection susceptibility, and three-fold higher than Delta infection susceptibility.

The study findings were in accordance with the United States (US) Centers for Disease Control and Prevention (CDC) report, which mentioned 3.5-fold higher hospital admission rates for 12- to 17-year-old individuals during the Omicron wave peak compared to those in the Delta predominance period. However, enhanced susceptibility to infection may not necessarily correlate with increased rates of hospital admission.

Studies have reported a shift in Omicron tropism from the lower respiratory tract to the upper respiratory tract, which is relatively smaller and immature in children compared to adults, which could explain the higher Omicron susceptibility in the pediatric population. Moreover, increased croup (acute laryngotracheobronchitis condition with characteristic barking cough), cases were detected during the period of Omicron predominance in the southern parts of Korea. Furthermore, Omicron prefers to invade host cells by endocytosis over the angiotensin-converting enzyme 2 (ACE2)-mediated pathway, which could further explain the increased proportion of pediatric Omicron infections as children possess lesser ACE receptors than adults.

Conclusion

To conclude, based on the study findings, children have increased susceptibility to Omicron infections compared to adults, and pediatric COVID-19 vaccinations may be considered to decrease the COVID-19 burden, although it is yet to be determined whether children are the key drivers of SARS-CoV-2 transmission.

*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

Pooja Toshniwal Paharia

Dr. based clinical-radiological diagnosis and management of oral lesions and conditions and associated maxillofacial disorders.