# Incidence of COVID-19-Associated Hospitalization by Vaccination Status in Children and Adolescents During Omicron-Dominant Period in Japan: The VENUS Study

## TL;DR

A study in Japan found that getting at least two doses of a COVID-19 vaccine reduced hospitalizations in children and adolescents during the Omicron wave.

## Contribution

This study provides real-world evidence of the effectiveness of ancestral monovalent vaccines in reducing hospitalizations in Japanese pediatric populations during the Omicron period.

## Key findings

- Fully vaccinated children had a 57% lower hospitalization rate compared to unvaccinated children (adjusted IRR 0.429).
- Gastrointestinal symptoms and dehydration were common reasons for hospitalization, with only 4.3% requiring oxygen administration.

## Abstract

What are the main findings?
Receipt of at least two doses of an ancestral monovalent COVID-19 vaccine was associated with a lower incidence rate of COVID-19-associated hospitalization among children and adolescents (adjusted IRR 0.429; 95% CI: 0.198–0.930).Only 4.3% of COVID-19-associated hospitalizations required oxygen administration, whereas 36.6% involved diagnoses related to vomiting, diarrhea, hypovolemia, or hypoglycemia, indicating that gastrointestinal symptoms and dehydration were common reasons for hospitalization.

Receipt of at least two doses of an ancestral monovalent COVID-19 vaccine was associated with a lower incidence rate of COVID-19-associated hospitalization among children and adolescents (adjusted IRR 0.429; 95% CI: 0.198–0.930).

Only 4.3% of COVID-19-associated hospitalizations required oxygen administration, whereas 36.6% involved diagnoses related to vomiting, diarrhea, hypovolemia, or hypoglycemia, indicating that gastrointestinal symptoms and dehydration were common reasons for hospitalization.

What are the implications of the main findings?
The results provide real-world evidence that pediatric COVID-19 vaccination was associated with a lower incidence rate of COVID-19-associated hospitalization in Japan during the Omicron-dominant period.Expanding COVID-19 vaccine uptake among eligible pediatric populations may reduce the future burden of severe disease.

The results provide real-world evidence that pediatric COVID-19 vaccination was associated with a lower incidence rate of COVID-19-associated hospitalization in Japan during the Omicron-dominant period.

Expanding COVID-19 vaccine uptake among eligible pediatric populations may reduce the future burden of severe disease.

Background/Objectives: Evidence regarding the impact of COVID-19 vaccination on children and adolescents remains limited, particularly in Japan. Thus, this study aimed to estimate incidence rates of COVID-19-associated hospitalization by vaccination status and evaluate the association between COVID-19 vaccination and COVID-19-associated hospitalization among individuals aged 6 months to <18 years in Japan. Methods: We conducted a retrospective, population-based cohort study using linked health insurance claims data and municipal COVID-19 vaccination registry records from nine Japanese municipalities between 1 January 2022, and 31 March 2023, when the Omicron variant was dominant nationwide. Incidence rates of COVID-19-associated hospitalization were estimated among unvaccinated individuals and those who had received one dose (partially vaccinated) or at least two doses of an ancestral monovalent COVID-19 vaccine (fully vaccinated). Incidence rate ratios (IRRs) were calculated, adjusting for age group, sex, presence of underlying medical conditions, municipality, and calendar month. Results: Among 163,305 individuals, 93 COVID-19-associated hospitalizations were identified. Crude incidence rates were 4.5 (95% confidence interval [CI]: 3.6–5.6), 3.2 (95% CI: 0.1–18.0), and 2.3 (95% CI: 1.1–4.2) per 100,000 person-months in unvaccinated, partially, and fully vaccinated groups, respectively. The adjusted IRR for the fully vaccinated group was 0.429 (95% CI: 0.198–0.930). Among hospitalized patients, 30.1% had underlying medical conditions, and only 4.3% required oxygen administration during hospitalization. Conclusions: Receiving at least two doses of COVID-19 vaccine was associated with a lower incidence rate of COVID-19-associated hospitalization among children and adolescents during the Omicron-dominant period in Japan. Expanding vaccine uptake among eligible pediatric populations may help reduce the burden of severe disease.