SARS-CoV-2 Omicron variant tends to spread more easily among children than prior strains

In a recent BMC Medicine journal study, researchers estimate the age-varying susceptibility to infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant.

To this end, an updated version of a previously described model with recent vaccine coverage data and waning vaccine effectiveness (VE) against the Omicron infection was used to estimate susceptibility to Omicron, particularly among zero to 19-year-olds, in South Korea.

Study: Identifying susceptibility of children and adolescents to the Omicron variant (B.1.1.529). Image Credit: Travelpixs / Shutterstock.com

Background

South Korea has experienced several severe waves of the coronavirus disease 2019 (COVID-19). Whereas the original SARS-CoV-2 strain caused the third COVID-19 wave, the Delta variant caused the fourth wave. More recently, the SARS-CoV-2 Omicron variant led to the fifth and most severe COVID-19 wave in January 2022.

South Korea has the National Infectious Disease Surveillance System (NIDSS), which compiles electronic reports of every COVID-19 diagnosis in the country. Thus, it is a suitable country to examine the age-specific susceptibility to the Omicron variant as compared to the Delta and pre-Delta strains using the epidemiologic data of three aforementioned COVID-19 waves and vaccine coverage.

The increasing proportion of pediatric COVID-19 cases globally since the emergence of Omicron is noteworthy. In the United States, the Omicron predominance era began in December 2021. Since then, the seroprevalence of infection-induced SARS-CoV-2 antibodies among children aged zero to 11 years increased from 44.2 to 75.2% between December 2021 and February 2022, the highest recorded increase among all age groups.

Although increased susceptibility to SARS-CoV-2 is not necessarily correlated with increased hospitalization rates, England also witnessed a three-fold rise in pediatric COVID-19-related hospital admissions in two weeks since December 26, 2021. Therefore, identifying the age-specific susceptibility to SARS-CoV-2 is of great interest for effective public health strategies and vaccination policy.

About the study

In the present study, researchers used the force of infection (λi) experienced by age group to estimate age-specific susceptibility. Moreover, a Bayesian inference method with a Markov chain Monte Carlo (MCMC) algorithm was used to estimate λi parameters.

The reproducibility of observed epidemiological patterns was confirmed using estimated case numbers and parameters. The MCMC algorithm inferred the exposure times when λi was known for each age group and then inferred the λi. These two steps were repeated until the Markov chain converged.

Study findings

The pre-Delta and Delta waves exhibited a similar age-dependent increase, whereas the Omicron wave was associated with an inverted bell curve with bimodal peaks.

The age-specific susceptibility among 0-19-year-olds was nearly five times higher during the Omicron wave than the third wave and three times higher than during the Delta wave. Indeed, children were more prone to Omicron infections than any other SARS-CoV-2 strains.

The rise in susceptibility to the Omicron and Delta variants was highest in 15-19 year-olds and lowest in those aged older than 75 years.

Omicron has shifted tropism to the upper respiratory tract (URT) from the lower respiratory tract; therefore, children whose URT is relatively small and underdeveloped compared to adults could be more easily affected by SARS-CoV-2. In addition, since children have fewer angiotensin-converting enzyme 2 (ACE2) receptors in the body, the preference of the endocytic pathway for entry by the Omicron variant as compared to the ACE2-dependent pathway may also contribute to their increased susceptibility to infection with this variant.

These factors increased Omicron cases in South Korea, where children also experienced a higher incidence of croup, an acute laryngotracheobronchitis characterized by a barking cough. Overall, these observations are consistent with both epidemiological and biological observations.

Conclusions

In this study, large-scale testing, epidemiological surveys, and vaccination status records maintained in a national registry in South Korea allowed researchers to analyze the age-stratified susceptibility to SARS-CoV-2. The SARS-CoV-2 Omicron variant was found to pose a greater risk of severe disease with increasing age and is more transmissible among children than the Delta and pre-Delta strains.

Regarding its age affinity, the SARS-CoV-2 Omicron variant might evolve like the influenza virus and eventually target the youngest and oldest people in a given population.

It remains unclear whether children could be the key drivers in SARS-CoV-2 transmission in the future. Nevertheless, vaccinating children against COVID-19 has the potential to reduce the pandemic’s impact on the community.