Abstract
The national influenza vaccination program for the 2023–2024 season targeted influenza immunization for children aged 6 months to 13 years, pregnant women, and seniors aged 65 years and older. This study aimed to analyze the primary outcomes of the national influenza vaccination program for the specified season. The vaccination rates for the identified groups were 82.5% for older adults and 53.0% for pregnant women, reflecting increases of 0.6 and 2.6 percentage points, respectively, compared with the corresponding values in the preceding season. Conversely, the vaccination rate for children declined by 1.5 percentage points, reaching 69.5%. There were 176 reported cases of adverse events following influenza vaccination, equaling 1.5 reports per 100,000 vaccinations. We intend to investigate the principal findings for the 2023–2024 season and use them as a foundation for improving the national influenza vaccination program in the future.
Keywords: Influenza, Immunization, Vaccination coverage
Key messages
① What is known previously?
Individuals aged 6 months to 13 years, pregnant women, and those aged 65 years and older are eligible for the national influenza vaccination program.
② What new information is presented?
The influenza vaccination rates among children, pregnant women, and older adults were 69.5%, 53.0%, and 82.5%, respectively. Compared with the previous season, in the 2023–2024 season, vaccination rates for older adults and pregnant women increased by 0.6 and 2.6 percentage points, respectively, whereas the rate for children declined by 1.5 percentage points.
③ What are implications?
Following the coronavirus disease 2019 pandemic, seasonal influenza viruses began circulating during the 2022–2023 season, highlighting the need to prevent infections in children, pregnant women, and older adults.
Introduction
Influenza is an acute respiratory viral disease, which is most prevalent seasonally from November to April. It can affect all age groups, with symptoms such as sudden fever, sore throat, cough, runny nose, and muscle aches. Older adults (aged 65 and above), young children, infants, and pregnant women are considered high-risk groups because of their higher morbidity and mortality rates from influenza infection [1].
In the Republic of Korea, a national influenza vaccination program for older adults aged 65 and above has been available through public health centers since 1997, and in 2015 the scope of the program was expanded to include private medical institutions under contract. Vaccination is known to reduce mortality by up to 80% among older adults aged 65 and above, making it crucial for preventing severe disease and death [1]. Similarly, influenza vaccination is effective in reducing the risk for severe and life-threatening influenza by 63% in children and 75% in adolescents [2]. Accordingly, the influenza vaccination program for children has expanded gradually: in 2016, eligibility included infants and children aged 6 months to under 12 months; in 2017, it extended to those aged 6 months to 59 months; by 2018, to children aged 6 months to 12 years; and in 2020, to those aged 6 months to 13 years [3]. For pregnant women, influenza vaccination reduces hospitalization by 40% [4] and offers 53% protection for infants under 3 months by passing antibodies to the fetus. Moreover, vaccination during the third trimester is 52% effective in protecting the child [5]. Consequently, the national influenza vaccination program for pregnant women has been in place since 2019.
During the 2020–2021 season, the influenza vaccination program was made available to older adults aged 62–64 years, adolescents aged 14–18 years, and recipients of disability pensions/allowances and medical aid. This was in preparation for the potential overlap of influenza and coronavirus disease 2019 (COVID-19) outbreaks during the pandemic.
This report reviews the key achievements of the national influenza vaccination program for the 2023–2024 season, including vaccination performance, and aims to provide baseline data for future operation of the program.
Main text
1. Overview of the National Influenza Vaccination Program for the 2023–2024 Season
For the 2023–2024 influenza vaccination season, children aged 6 months to 13 years and older adults aged 65 and above were targeted. The vaccination period was selected considering the influenza season and the fact that antibodies are formed approximately two weeks after vaccination and lasts for approximately six months (Table 1). For children, the program began in September, allowing sufficient time for them to acquire adequate immunity through two doses (with at least four weeks between doses). For older adults, the vaccination period was adjusted according to age to ensure safe administration, as vaccinations were concentrated in the early phase of the program.
Table 1. Vaccination target and period, 2023–2024 season .
| Vaccination target | Vaccination period | |
|---|---|---|
| Children (Born in Jan. 1, 2010–Aug. 31, 2023) | Two doses of flu vaccinea) | 2023.9.20.–2024.4.30. |
| One doses of flu vaccine (under 13 yr) | 2023.10.5.–2024.4.30. | |
| Pregnant women | 2023.10.5.–2024.4.30. | |
| Elderly people (Born before Dec. 31, 1958) | Over 75 yr | 2023.10.11.–2024.4.30. |
| 70–74 yr above | 2023.10.16.–2024.4.30. | |
| 65–69 yr above | 2023.10.19.–2024.4.30. | |
a)Under the age of 9 who have received influenza vaccination for the first time or who have received a total of less than two doses by June 30, 2023.
Vaccinations were provided at 22,400 locations nationwide, including at public health centers, health subcenters, primary health care posts, and contracted medical institutions. Eligible individuals could receive government-subsidized quadrivalent influenza vaccine by visiting any of the listed institutions, regardless of their place of residence. This season’s program aimed to achieve vaccination rates of 82% for older adults, 55% for pregnant women, and 75% for children (single dose and first of two doses).
The vaccination rate was calculated using the Integrated Disease Health Management System of the Korea Disease Control and Prevention Agency. Among the individuals registered in the Integrated Vaccination Management System, those born in the applicable year were identified. Immunization records were compiled by contracted medical institutions and public health centers, and vaccination rates were calculated according to these electronic records. The vaccination rate among children was calculated on the basis of the percentage of children receiving a single dose and those completing the first of two doses. The vaccination rate for pregnant women was estimated using publicly available data on the number of children born in 2022, given the challenge of identifying the actual number of eligible individuals for that season. Both government-subsidized and out-of-pocket vaccinations were included in the calculation of vaccination cases and rates.
2. Performance of Influenza Vaccination for Older Adults
Among 9,766,851 older adults eligible for the influenza vaccination program, 8,059,311 were vaccinated, representing a vaccination rate of 82.5%, which was an increase of 0.6% from 81.9% in the previous season (Figure 1).
Figure 1. Number of vaccinations elderly people compared vaccination coverage to last season.
For older adults, vaccinations are typically concentrated in the early stages of the program. Thus, to ensure safety, the vaccination period was divided into three based on age groups, with the oldest group receiving vaccinations first.
Regarding vaccination institutions, 4.4% of vaccinations were administered at public health centers (n=354,477), and 95.6% at contracted medical institutions (n=7,704,834), showing a slight increase in vaccinations at contracted medical institutions compared with the previous season’s figures of 4.6% at public health centers and 95.3% at contracted medical institutions (Table 2). In terms of vaccination rates among older adults by region, North Jeolla Province had the highest rate (85.5%), followed by South Jeolla Province (85.3%) and North Chungcheong Province (84.6%). Gyeonggi Province (n=2,129,402) and Seoul (n=1,727,198) had the highest number of vaccinations administered.
Table 2. Number of vaccinations by health services .
| Characteristics | No. of population |
No. of vaccinated | Vaccine coverage |
|||
|---|---|---|---|---|---|---|
| Total | Public health center |
Medical institution |
||||
| Elderly people | 2023–2024 season | 9,766,851 | 8,059,311 (100.0%) | 354,477 (4.4%) | 7,704,834 (95.6%) | 82.5% |
| 2022–2023 season | 9,310,653 | 7,629,522 (100.0%) | 357,603 (4.6%) | 7,271,919 (95.3%) | 81.9% | |
| Pregnant women | 2023–2024 season | 253,598 | 134,357 (100.0%) | 599 (0.4%) | 133,758 (99.6%) | 53.0% |
| 2022–2023 season | 265,262 | 133,735 (100.0%) | 543 (0.4%) | 133,192 (99.6%) | 50.4% | |
| Children | ||||||
| 2023–2024 season | 5,127,904 | 3,562,735a) (100.0%) | 29,832a) (0.8%) | 3,532,903a) (99.2%) | 69.5%a) | |
| 2022–2023 season | 5,333,556 | 3,785,738a) (100.0%) | 32,669a) (0.9%) | 3,753,069a) (99.1%) | 71.0%a) | |
Values are presented as number (%). a)1 Shot and first vaccination of 2 shot.
Since the concurrent administration of influenza and COVID-19 vaccines is allowed and a part of the program period overlapped, the rate of concurrent vaccination among older adults was also examined. The population of older adults who received both vaccines concurrently was identified through the COVID-19 Vaccination Management System and Integrated Vaccination Management System, using data from the Integrated Disease Health Management System. Among eligible older adults, a total 991,969 (10.2%) received both vaccines, representing 12.3% of those who received the influenza vaccine. This marks a more than three-fold increase compared with the previous season (n=308,117), likely due to recommendations encouraging concurrent vaccination for the 2023–2024 season.
3. Performance of Influenza Vaccination for Pregnant Women
Influenza vaccination for pregnant women has been available since 2019. In the 2023–2024 season, 134,357 out of 253,598 eligible pregnant women were vaccinated, resulting in a vaccination rate of 53%. This represents an increase of 2.6% from the previous season’s rate of 50.4%. Regarding the vaccination institutions, 0.4% of vaccinations were administered at public health centers (n=599), while 99.6% occurred at contracted medical institutions (n=133,758), which is consistent with the previous season (Table 2). In terms of vaccination rates among pregnant women by region, Ulsan had the highest rate (62.5%), followed by Seoul (58.0%) and Gangwon Province (57.9%). Meanwhile, Gyeonggi Province (n=77,910) and Seoul (n=40,608) reported the highest number of vaccination cases.
4. Performance of Influenza Vaccination for Children
For the 2023–2024 season, 3,562,735 out of 5,127,904 children were vaccinated, resulting in a vaccination rate of 69.5% (single dose and first of two doses) (Table 2). This represents a decrease of 0.6% from the previous season’s rate of 71.0% (Figure 2).
Figure 2. Number of vaccinations children compared vaccination coverage to last season.
Vaccination rates by age were 82.5, 75.7%, 68.8%, 61.6%, and 49.2% for children aged 6–59 months, 60–83 months, 7–9 years, 10–12 years, and 13 years, respectively. Vaccination rates were higher in younger age groups, similar to the previous season. In terms of vaccination institutions, among a total of 3,797,079 vaccination cases (including single doses and first and second doses), 31.748 cases (0.8%) were administered at public health centers, while 3,765,331 cases (99.2%) occurred at contracted medical institutions. By region, Incheon had the highest rate (72.8%), followed by South Chungcheong Province (72.3%) and Gyeonggi Province (70.9%). For specific age groups, the highest rates were found in Incheon for children aged 6–59 months (84.7%), Incheon and South Chungcheong Province for children aged 60–83 months (78.7%), Incheon for children aged 7–9 years (72.3%), South Chungcheong Province for children aged 10–12 years (65.0%), and South Chungcheong Province for children aged 13 years (53.3%).
5. Reports of Adverse Events after Influenza Vaccination
During the 2023–2024 season, 176 cases of adverse events after influenza vaccination were reported, representing a slight increase from the previous season, but the number of reported cases per 100,000 was 1.5 (105, 1,626, 108, and 118 cases in 2019–2020, 2020–2021, 2021–2022, and 2022–2023 seasons, respectively). The number of adverse event cases reported among children, pregnant women, and older adults was 41 (44 in previous season), 38 (0 in previous season), and 97 (74 in previous season), respectively (Table 3).
Table 3. Types of adverse events, 2023–2024 season .
| Characteristics | Total (A=B+C) | General adverse events (B) |
Severe adverse events | No. of vaccinated (D) |
Adverse events rate per 100 thousand (E=A/D*105) | |||
|---|---|---|---|---|---|---|---|---|
| Total (C=C1+C2+C3) | Death (C1)a) | Suspected anaphylaxis (C2) | Major adverse events (C3)b) |
|||||
| Total | 176 | 166 | 10 | 4 | 1 | 5 | 11,756,403 | 1.50 |
| Elderly people | 97 | 89 | 8 | 4 | 0 | 4 | - | - |
| Pregnant women | 38 | 38 | 0 | 0 | 0 | 0 | - | - |
| Children | 41 | 39 | 2 | 0 | 1 | 1 | - | - |
-=not available. (A) Accumulated period: September 20, 2023 to April 30, 2024 (It was calculated based on information reported by medical institutions or public health centers as a suspected adverse event after vaccination, and does not suggest causality between vaccines and adverse events. Report status classification may be changed when new information is added). (B) Common adverse events include common symptoms such as redness, pain, swelling, myalgia, fever, headache, chills after vaccination. (C) Severe adverse event. ① Death, ② Suspected anaphylaxis, ③ Major adverse events: causality assessment for serious AEFI etc. a)Death: cardiac arrest (1), etc (3). b)Major adverse events: sepsis (2), Guillain-Barre syndrome (1), encephalopathy/encephalitis (1), brachial plexopathy (1).
The types of adverse events reported after vaccination included 166 cases of general adverse events (94.3%); 10 cases of serious adverse events, including four deaths (2.3%); one case of suspected anaphylaxis (0.6%); and five cases requiring admission into intensive care (2.8%).
Conclusions
The national influenza vaccination program for the 2023–2024 season was available for children aged 6 months to 13 years, pregnant women, and older adults aged 65 and older. Vaccination rates among older adults and pregnant women increased compared with the previous season. In contrast, the vaccination rate among children decreased, which can be attributed to early outbreaks in this demographic during a season when influenza cases reached their highest number in the past five years.
Although there have not been major influenza epidemics since the COVID-19 pandemic, a pattern of seasonal epidemics began during the 2022–2023 season [6]. The percentage of suspected cases during the 2023–2024 season remained high among school-aged children compared with previous seasons. Therefore, it is essential to prepare for the national influenza vaccination program for the 2024–2025 season by enhancing management for safe vaccinations and implementing targeted promotions to encourage influenza vaccination among different populations.
Acknowledgments
None.
Supplementary Materials
Supplementary data are available online.
Declarations
Ethics Statement: Not applicable.
Funding Source: None.
Conflict of Interest: The authors have no conflicts of interest to declare.
Author Contributions: Conceptualization: SHA, WKL. Data curation: SHA, HWH, MJK, YRK, JYL. Methodology: SHA, HWH, MJK. Supervision: WKL, DWL, YRK. Writing – original draft: SHA, MJK, WKL. Writing – review & editing: SHA, WKL, DWL.
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