PHWR Public Health Weekly Report

Vaccination is the most cost-effective strategy for the prevention of infectious diseases in targeted populations, yielding effectiveness of approximately 16 times greater than the investment in disease prevention [1]. In the Republic of Korea (ROK), the National Immunization Program (NIP) for the prevention of vaccine-preventable infectious diseases was launched in 1954, in accordance with Article 24 and Article 25 of the Infectious Disease Control and Prevention Act. To achieve herd immunity through an immunization rate of 95% or higher for mandatory vaccinations, such as measles [2], the NIP has been expanded to include public health centers to provide free vaccinations from 2014. This has reduced the financial burden on citizens with respect to vaccination costs and improved the convenience of vaccination. Since March 2023, free rotavirus vaccinations, which cost approximately 200,000 to 300,000 KRW, have been offered at no cost. Through this program, the government offers free vaccinations for 18 vaccines. In addition, Immunization Registry Information System (IRIS) has been established to effectively manage non-vaccinated individuals and vaccination records. The system provides individualized vaccination information and vaccination schedules, as well as delayed vaccination notifications based on the collected data to continuously manage non-vaccinated individuals, and the quality of the collected data is continuously monitored. The Korea Disease Control and Prevention Agency (KDCA) is working on the enhancement of the accuracy of vaccination information by identifying and improving the status of erroneous data in the vaccination registration data, including personal information and duplicate or missing vaccination information.

Approved statistics serve as a crucial metric for assessing the effectiveness of the NIP. Policies to prevent vaccine-preventable infectious diseases are developed based on these data, and the response strategies are devised according to the immunization rate.

In this article, we aim to describe the data collection, computation, and analysis methods and results for immunization rates among children aged 1–3 years and 6 years in 2022. Additionally, we will discuss the limitations of calculating the vaccination rates and outline the directions for improving the rates of vaccination.

1. Vaccination Data Collection and Computation

The vaccination records were obtained from IRIS, a KDCA system onto which public health centers and private medical institutions record vaccination data. To ensure the accuracy of electronically registered data, various measures were implemented, including linking of the data with the Ministry of the Interior and Safety’s birth, death, and address information, as well as performing quality control, including the verification of personal information, checking for vaccination errors, management of cases of missing registrations, and tracking of non-vaccinated individuals. After conducting quality control checks and ensuring the accuracy of the information collected through the system, the KDCA analyzed the data. The final immunization rates were confirmed based on the advice of a panel of vaccination statistics experts during a consultation held in June 2023 (Figure 1).

Figure 1. Flow of vaccination coverage data collection and analysis
KDCA=The Korea Disease Control and Prevention Agency.

The nationwide immunization rate in children is based on comprehensive survey statistics derived from the electronic registry data from IRIS for Korean citizens aged 1–3 years and 6 years as of the end of the respective year.

The target population for the analysis of the nationwide immunization rate in children in 2022 was defined as Korean nationals residing in the ROK who were aged 1–3 and 6 years old (born in 2016, 2019–2021 subject to vaccination), and children who had passed away or resided abroad were excluded. The full immunization rate was calculated from children who are subject to vaccination and have completed all recommended doses for their ages per the standard vaccination schedule in accordance with the Immunization Guidelines and Methods (KDCA Bulletin 2023–5). The rate was calculated based on the records updated in the system until the end of June 2023.

Vaccines given as combination vaccines, such as DTaP-IPV, DTaP-IPV/Hib, DTaP-IPV-HepB-Hib, each vaccine included in the combination (diphtheria-tetanus-acellular-pertussis [DTaP], inactivated polio vaccine [IPV], Haemophilus influenza type b [Hib], hepatitis B [HepB]) was considered separately. The records of vaccinations administered earlier than the minimum vaccination age, minimum interval between vaccinations, or the minimum interval between live attenuated vaccines (4 weeks) recommended in the Immunization Guidelines were excluded. In cases of duplicate administration of the same vaccination, only the latest vaccination was used. For certain vaccines (Hib, pneumococcal conjugate vaccine [PCV], DTaP, IPV, Japanese encephalitis [JE] inactivated vaccine), vaccinations were recognized even when the recommended doses were not fully completed with delayed vaccination if the criteria set forth in the “Immunization Guidelines and Methods” (KDCA notice 2023-5) were met, such as adhering to the catch-up schedules [3].

1. Immunization Rate

As of the end of 2022, the sex- and age-specific immunization rates for 1.26 million children born in 2016 and 2019–2021 registered on the Ministry of the Interior and Safety resident registration system are shown below (Table 1).

Table 1 . Subjects of birth cohorts used as denominator, 2022.

	Total number of children	Male	Female
12 months of age (born in 2021)	330,836	169,753	161,083
24 months of age (born in 2020)	362,592	186,676	175,916
36 months of age (born in 2019)	411,665	210,643	201,022
72 months of age (born in 2016)	440,762	225,781	214,981

Unit: n..

By birth year, the full immunization rate¹⁾ was 96.1% for 1-year-old (born in 2021, 6 vaccines, 16 doses)²⁾, 93.5% for 2-year-old (born in 2020, 8 vaccines, 21 doses)³⁾, 89.7% for 3-year-old (born in 2019, 10 vaccines, 25–26 doses)⁴⁾, and 88.6% for 6-year-old (born in 2016, 10 vaccines, 28–30 doses)⁵⁾ (Figure 2).

Figure 2. Vaccination coverage by 12, 24, 36 and 72 months of age
BCG=Bacille Calmette-Gúerin; HepB=Hepatitis B; DTaP=Diphtheria-Tetanus-acellular-Pertussis; IPV=Inactivated Polio Vaccine; Hib= Haemophilus influenza type b; PCV=Pneumococcal Conjugate Vaccine; MMR=Measles-Mumps-Rubella; VAR=Varicella; HepA=Hepatitis A; JE=Japanese Encephalitis.

By vaccine, the immunization rate for ages 1, 2, 3, and 6 years ranged from 91.9% to 97.8%. The immunization rate was 95% or higher for all types of vaccines, with the exception of DTaP and JE. Overall, the immunization rates were high for vaccines administered before the age of 1 year, such as Bacille Calmette-Gúerin (BCG) and HepB, as well as vaccines with few dose frequencies, such as varicella (VAR) and Measles-Mumps-Rubella (MMR), ranging from 97% to 98% (Figure 2).

By region, the immunization rates were high in Ulsan and Sejong and lowest in Seoul in all age groups. By birth year, the immunization rate for 1-year-olds was 95% or above in all regions, except Seoul. The immunization rate was the lowest in Seoul, followed by Gwangju and Jeonbuk in 2-year-olds, lowest in Gwangju in 3-year-olds, and lowest in Seoul and Jeju in 6-year-olds. The individual vaccination rates also showed similar trends as the overall immunization rate (Figure 3).

Figure 3. Vaccination coverage by age and region 2022

The full immunization rate for the six vaccines (DTaP, IPV, MMR, VAR, Hib, and PCV) among 2-year-old in the ROK is 96.5%, which is approximately 2–10%p higher than that in other major countries that use the same vaccination criteria, such as the United States (86.6%), Australia (94.7%), and United Kingdom (91.3%), showing that ROK maintains a high level of immunity in children globally. Similarly, the immunization rates for each vaccine were higher in the ROK compared to other countries (Figure 4) [4,-6].

Figure 4. Vaccination coverage by country
(A) Vaccination coverage by age 24 months among children born during 2018–2019; National Immunization Survey-child, United States, 2019–2021. (B) National Statistics. Childhood Vaccination Coverage Statistics, England, 2021–2022, 2022. (C) NSW Annual Immunisation Coverage Report, 2021. DTaP=diphtheria-tetanus-acellular-pertusis; IPV=inactivated polio vaccine; MMR=measles-mumps-rubella; VAR=varicella; Hib=Haemophilus influenza type b; PCV=pneumococcal conjugate vaccine; NSW=New South Wales.

1. Limitations in the Calculation of Immunization Rate

The statistics were collected based on the residential addresses recorded in the resident registration system as of the end of 2022; therefore, there may be some discrepancies from the immunization rates among children currently residing in the community. Additionally, children who are confirmed to be residing abroad or have been out of the country for long periods of time are excluded from the target population for immunization. However, children with unconfirmed statuses are included, which could result in lower calculated immunization rates in regions where the proportion of such children is high. Furthermore, for certain vaccines that are not included in the NIP, but are included in the calculation of immunization rates, such as BCG (for percutaneous), JE inactivated vaccines, and DTaP-IPV-HepB-Hib, the calculation of immunization rates is based on voluntarily reported data. This may lead to an underestimation of the rate due to factors, such as omissions in the computer system.

2. Interpretation of Immunization Rate and Directions for Improvement

The immunization rates tended to decrease with advancing age primarily due to the increasing number of vaccines and doses required for completion. Among the analyzed age groups, 6-year-old had the lowest immunization rates. In this age group, the DTaP immunization rate (requires 4–5 doses) was low, and the JE inactivated immunization rate appears to have been the lowest due to the delayed first dose (12 months), third dose (24–35 months), and fourth booster dose (6 years) compared to other vaccines.

Furthermore, the immunization rates were low in regions with higher percentages of children residing abroad due to reasons, such as dual citizenship and long-term out-of-country stay [7], and the rates are anticipated to differ also according to efforts made by local governments, such as vaccination promotions through mail and text messages and frequencies of promotions.

This year, the immunization rates for children aged 1–3 years remained relatively stable or showed a slight decrease over the 3 years leading up to the coronavirus disease 2019 (COVID-19) pandemic (2020–2022). However, the full immunization rate for 6-year-olds increased by 5.1%p during the same period (from 83.5% in 2020 to 88.6% in 2022). Maintaining high immunization rates during the COVID-19 pandemic can be attributed to active participation in disease prevention nationwide and the resumption of the elementary school immunization verification program⁶⁾ in collaboration with the Ministry of Education [8]. Nonetheless, there was a slight downward trend in the immunization rates for ages 1–3 years, calling for vaccination strategies specifically tailored to this age group, similar to the 6-year-old group. Given the success of the elementary school immunization verification program, it is necessary to strengthen the coordination of immunization information with daycare and preschools, where children aged 1–3 years spend most of their time, as well as other facilities and increase the vaccination promotions.

Considering that vaccination is the most effective method for preventing and managing infectious diseases, immunization rates are valuable indicators for infectious disease management planning. Immunization rates serve as crucial metrics for assessing the performance of the NIP, allowing for the identification of issues of the current projects and the development of local response strategies at the si, gun, and gu levels. Additionally, a high full immunization rate suggests that herd immunity has been achieved to prevent outbreaks of infectious diseases that spread through group living; thus, the management of immunization rates is extremely important.

While the ROK maintains a high level of immunization rates compared to other countries, disparities exist in the immunization rates for certain vaccines and by region. To address these disparities, it is essential to analyze the unvaccinated population, develop tailored promotional strategies to improve the immunization rates, and provide accurate information about vaccines.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: JYL, SLK, HML. Data curation: JYL. Formal analysis: JYL, SLK, HML. Investigation: JYL. Methodology: JYL, SLK, HML. Project administration: SLK, HML. Resources: JYL. Software: JYL. Supervision: SLK, HML. Validation: JYL, SLK, HML. Visualization: JYL, SLK, HML. Writing – original draft: JYL, SLK, HML. Writing – review & editing: SLK, HML.

1) Full immunization rate: Percentage of children who have received all recommended vaccine doses according to age-specific vaccination schedules.

2) 6 vaccines: BCG 1 dose, HepB 3 doses, DTaP 3 doses, IPV 3 doses, Hib 3 doses, and PCV 3 doses.

3) 8 vaccines: BCG 1 dose, HepB 3 doses, DTaP 4 doses, IPV 3 doses, Hib 4 doses, PCV 4 doses, MMR 1 dose, and VAR 1 dose.

4) 10 vaccines: BCG 1 dose, HepB 3 doses, DTaP 4 doses, IPV 3 doses, Hib 4 doses, PCV 4 doses, MMR 1 dose, VAR 1 dose, HepA 2 doses, and JE inactivated vaccine 3 doses or JE live attenuated vaccine 2 doses.

5) 10 vaccines: BCG 1 dose, HepB 3 doses, DTaP 5 doses, IPV 4 doses, Hib 4 doses, PCV 4 doses, MMR 2 doses, VAR 1 dose, HepA 2 doses, and JE inactivated vaccine 4 doses or JE live attenuated vaccine 2 doses.

6) Elementary school vaccination verification program: this program verifies newly admitted students’ booster vaccination status at age 4–6 years within 90 days of elementary school admission (DTaP 5th dose, IPV 4th dose, MMR 2nd dose, JE inactivated vaccine 4th dose, or JE live attenuated vaccine 2nd dose) and encourages vaccination for unvaccinated children to prevent infectious diseases and protect student health.

Supplementary data are available online.