PHWR Public Health Weekly Report

Influenza is an infection caused by the influenza virus characterized by systemic symptoms, such as sudden fever (≥38^℃), muscle pain, headache, and respiratory symptoms such as sore throat and cough. The flu season generally persists from November to April of the following year, and older adults aged ≥65 years, children <5 years, and individuals with chronic diseases are at an elevated risk for complications, hospitalization, and mortality [1].

In 1997, the Republic of Korea (ROK) launched a national influenza vaccination campaign for older adults aged ≥65 years through public health centers; the campaign expanded to include private designated healthcare facilities in 2015. In addition, the National Child Influenza Vaccination Campaign initially targeted infants aged 6–11 months in 2016 and was eventually expanded to include children aged 6–59 months in 2017, children aged 6 months to 12 years in 2018, and children aged 6 months to 13 years in 2020 [2]. As pregnant women are at high risk for complications, such as pneumonia, once they develop an influenza infection, they were included in the national campaign from 2019 to protect fetuses and infants <6 months.

In the 2020–2021 season, the target population of influenza vaccination temporarily extended to older adults aged 62–64 years, adolescents aged 14–18 years, and individuals receiving disability benefits or medical aid, accommodating the concurrent epidemic of influenza and coronavirus disease 2019(COVID-19).

This article reviews the major outcomes of the 2022–2023 season national influenza vaccination campaign in the ROK and presents valuable data for subsequent national influenza vaccination campaigns in the country.

1. Outline of the 2022–2023 Season National Influenza Vaccination Campaign

The 2022–2023 season national influenza vaccination campaign targeted older adults aged ≥65 years, pregnant women, and children aged 6 months to 13 years. The campaign length was established considering the antibody production against the influenza virus from approximately 2 weeks after the vaccination, lasting approximately 6 months, and the timing of the flu epidemic in the previous season (Table 1). The National Child Influenza Vaccination Campaign was launched in September, as children require two doses to acquire immunity (two doses with a 4-week interval), and older adults are targeted early in the campaign. Thus, vaccination timing was determined by age group for ensuring safe vaccination.

Table 1 Vaccination target and period, 2022–2023 season

Vaccination target	Vaccination period
Children (born in Jan. 1, 2009–Aug. 31, 2022)
Two doses of flu vaccinea)	2022.9.21.–2023.4.30.
One doses of flu vaccine (under 13 years of age)	2022.10.5.–2023.4.30.
Pregnant women	2022.10.5.–2023.4.30.
Older adults (born before Dec. 31, 1957)
Over 75 yr	2022.10.12.–2022.12.31.
70–74 yr above	2022.10.17.–2022.12.31.
65–69 yr above	2022.10.20.–2022.12.31.

^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, 2022..

Regardless of residence, free flu vaccines are provided at 21,930 public health centers, public health branches, public health clinics, and contracted healthcare facilities nationwide. Unlike the Influenza trivalent vaccine, the quadrivalent flu vaccine was chosen as it contains one more B-type virus antigen, reducing the possibility of mismatch with the prevailing influenza strain and providing excellent influenza prevention [3].

The vaccination rate was calculated based on birth year among those registered in the Comprehensive Vaccination Management System of the Integrated Disease Prevention and Health Promotion Management System of the Korea Disease Control and Prevention Agency. Electronic vaccination records were updated by public health centers or contracted healthcare facilities. The child vaccination rate was estimated among those who completed the first vaccination dose and were eligible for one or two doses. Owing to the difficulty confirming the target population in pregnant women, the vaccination rate based its calculation on the number of births in 2021 as a substitute. Nationally supported and voluntarily administered vaccinations were included when calculating vaccination counts and rates.

2. Influenza Vaccination Rates among Older Adults

Of 9,310,653 older adults eligible for the free flu vaccine, 7,629,522 received the vaccination (rate, 81.9%), a 1.4%p increase from the 80.5% in the previous season.

Regarding the vaccination facility, 4.7% (n=357,603) received the vaccination from a public health centers, while 95.3% (n=7,271,919) received it from a contracted healthcare facility, showing a slight increase in the percentage of individuals receiving the vaccine at a contracted healthcare facility compared to the previous season (4.9% and 95.1%, respectively; Table 2). By region, the vaccination rate among older adults was the highest in Jeonnam (84.6%), followed by Jeonbuk (84.5%) and Chungbuk (83.9%), and the number of vaccines administered was high in Gyeonggi (1,630,188 cases) and Seoul (1,343,754 cases).

Table 2 Number of vaccinations of older adults and pregnant women, by health services

Characteristics	No. of population	No. of vaccinated			Vaccine coverage (%)
		Total	Public health center	Medical institution
Older adults
2022–2023 season	9,310,653	7,629,522 (100)	357,603 (4.7)	7,271,919 (95.3)	81.9
2021–2022 season	8,866,005	7,133,922 (100)	346,062 (4.9)	6,787,860 (95.1)	80.5
Pregnant women
2022–2023 season	265,262	133,735 (100)	543 (0.4)	133,192 (99.6)	50.4
2021–2022 season	275,209	149,226 (100)	397 (0.3)	148,829 (99.7)	54.2

Vaccination for older adults was concentrated early in the campaign, where 5,134,231 (55.2% of eligible individuals) older adults received the vaccine in the first 2 weeks (Figure 1). As with previous seasons, vaccinations in the 2022–2023 season were instituted by age groups in three phases to prevent overcrowding in vaccination clinics and to ensure patient safety accounting for the COVID-19 pandemic. This phased vaccination seems effective, as evidenced by the 978,101 aged ≥75 years receiving vaccines on the first day of vaccination (October 12), 740,223 aged ≥70 years on first day (October 17), and 711,325 aged ≥65 years on first day (October 20).

Figure 1. Number of vaccinations older adults by week and compared vaccination coverage to last season
^a)Week 1: 2022.10.12.–2022.10.16.

3. Influenza Vaccination Rates among Pregnant Women

Pregnant women were included in the national influenza vaccination campaign in 2019. In the 2022–2023 season, 133,735 of 265,262 eligible women received the vaccine (rate 50.4%), a 3.8%p decrease compared to 54.2% in the previous season. Concerning the vaccination facility, 0.4% (n=543) received the vaccination from a public health centers, while 99.6% (n=133,192) received the vaccination from a contracted healthcare facility, similar to the percentages in the previous season (0.3% and 99.7%, respectively; Table 2). Regarding the region, the vaccination rate among pregnant women was the highest in Gangwon (56.5%), followed by Daejeon (54.1%) and Seoul (53.5%), while the number of vaccines administered was high in Gyeonggi (40,527 cases) and Seoul (23,228 cases).

4. Influenza Vaccination Rates among Children

In the 2022–2023 season, the influenza vaccination rate among children was 71.0%; 3,785,738 of 5,333,556 eligible children received the vaccination (including single dose and first dose from a two-dose series) (Table 3), which is a 2.8%p decrease from 73.8% in the previous season (Figure 2).

Figure 2. Number of vaccinations children by week and compared vaccination coverage to last season
^a)Week 1: 2022.9.21.–2022.9.25.

Table 3 Vaccination coverage of children, 2022-2023 season

Characteristics	No. of population	No. of vaccinated			Vaccine coverage (%)
		Totalc)	Free	Paid	Totalc)	Free	Paid
2022–2023 season
Totala)	5,333,556	3,785,738	3,757,015	28,723	71.0	70.4	0.5
1 shot (younger 13 yr)	4,908,767	3,513,320	3,485,534	30,261	71.6	71.0	0.6
2 shotb)	424,789
1st		272,418	271,481	937	64.1	63.9	0.2
2nd		242,974	242,230	744	57.2	57.0	0.2
2021–2022 season
Totala)	5,544,747	4,091,520	4,048,741	42,779	73.8	73.0	0.8
1 dose (younger 13 yr)	5,083,764	3,792,340	3,750,729	41,611	74.6	73.8	0.8
2 doseb)	460,983
1st		299,180	298,012	1,168	64.9	64.6	0.3
2nd		265,081	263,941	1,140	57.5	57.3	0.2

^a)1 shot and first vaccination of 2 shot. ^b)No. of people who have received less than two doses of influenza in previous season (vaccination registered in the Integrated Vaccination Management System). ^c)Including national vaccinations and paid vaccinations..

The single-dose vaccination rate was 83.9% in 6–59-month-olds, 79.2% in 60–83-month-olds, 72.7% in 7–9-year-olds, and 61.0% in 10–13-year-olds, showing an increasing vaccination rate with decreasing age, as observed in the previous season. In relation to the vaccination clinic, out of 4,028,712 total cases (including single dose and first dose from a two-dose series), 34,768 vaccines (0.9%) were received at a public health centers, while 3,993,944 vaccines (99.1%) were received at a contracted healthcare facility. Considering region, the vaccination rate among children was the highest in Incheon (74.7%), followed by Chungnam (73.7%) and Gyeonggi (72.8%). Furthermore, the vaccination rate was the highest in children aged 6–35 months (90.0%), followed by 36–59 months (82.4%), 60–83 months (81.7%), 7–9 years (75.7%), and 10–13 years (64.9%). The vaccination rate was the highest in Incheon among all age groups.

5. Reports of Adverse Reactions after Influenza Vaccination

Overall, 118 cases of adverse reactions were reported following influenza vaccination in the 2022–2023 season (1.0 cases per 100,000 vaccination cases), similar to the number reported in previous years (105 cases in the 2019–2020 season, 1,626 cases in the 2020–2021 season, and 108 cases in the 2021–2022 season). Adverse reactions were reported in 74 cases among older adults (54 in the previous year), 0 cases reported among pregnant women (20 in the previous year), and 44 cases reported among children (34 in the previous year; Table 4).

Table 4 Types of adverse reactions, 2022–2023 season

Characteristics	Total(A=B+C)	General adverse reactions (B)	Severe adverse reactions				No. of vaccinated (D)	Adverse reactions rate per 100 thousand (E=A/D)
			Total(C=C1+ C2+C3)	Death (C1)	Suspected anaphylaxis (C2)	Majoradverse reactions (C3)
Total	118	109	9	6a)	1	2b)	11,848,067	1.0
Older adults	74	66	8	6	0	2
Pregnant women	0	0	0	0	0	0
Children	44	43	1	0	1	0

A, accumulated period: July 1, 2022 to April 30, 2023 (it was calculated based on information reported by medical institutions or public health centers as a suspected adverse reaction after vaccination, and does not suggest causality between vaccines and adverse reactions. Report status classification may be changed when new information is added). B, common adverse reactions include common symptoms such as redness, pain, swelling, myalgia, fever, headache, chills after vaccination. C, severe adverse reaction. ① death, ② suspected anaphylaxis (including anaphylaxis-like reaction), ③ major adverse reactions: intensive care unit (ICU) admission, critical life, permanent disability/ aftermath etc. ^a)Death: dyspnea (1), Guillain-Barre syndrome (1), acute cardiac death (1), sepsis shock (2), intraperitoneal bleeding (1), ^b)ICU admission: sepsis shock (2)..

The types of adverse reactions included general adverse reactions (109 cases, 92.4%), six cases of death from nine serious adverse events (5.1%), suspected anaphylaxis (one case, 0.8%), and intensive care unit admission (two cases, 1.7%). After epidemiological investigations and injury compensation reviews of the serious adverse events, one case of anaphylaxis was deemed to have causal relevance to the vaccine, while the remaining five cases were determined to have some temporal relationship with the vaccine but were more likely to have been caused by other reasons, such as underlying disease; three of these cases were determined to have no causal relationship with the vaccine.

6. Reports of Adverse Reactions after Concurrent Influenza and COVID-19 Vaccination

In the 2022–2023 season, the national influenza vaccination campaign partially overlapped with the COVID-19 vaccination campaign. As co-administration of the flu vaccine and COVID-19 vaccine is permitted, we compared the COVID-19 adverse reaction rate and adverse reaction rate among individuals who received both vaccines on the same day. Based on records from the COVID-19 vaccination management system and the comprehensive vaccination management system of the Integrated Disease Prevention and Health Promotion Management System, 308,117 individuals were found to have received both vaccines on the same day (Table 5). This accounts for 2.60% of all individuals who received the flu vaccine. Collectively, three cases of adverse reactions to the flu vaccine and 105 cases of adverse reactions to the COVID-19 vaccine were reported. The adverse reaction rate among individuals who received both vaccines on the same day was 0.035%. Additional research is required to determine whether co-administration of the vaccines contributed to the adverse reaction reporting rate.

Table 5 Coadministration of influenza and COVID-19 vaccines adverse reactions, 2022–2023 season

Characteristics	No. of adverse reactions (A)	No. of vaccinated (B)	Adverse reactions rate (C=A/B×100)
Total COVID-19	2,606	10,397,058	0.025
Coadministration (COVID-19+influenza)	108	308,117	0.035
Influenza	118	11,848,067	0.001

COVID-19=coronavirus disease 2019..

In the 2022–2023 season, the national influenza vaccination campaign targeted children aged 6 months to 13 years, pregnant women, and older adults aged ≥65 years. The vaccination rate increased compared to the previous season among older adults; contrarily, it decreased among pregnant women and children. In the two seasons preceding the 2022–2023 season, the proportion of influenza-like illness cases remained below the epidemic threshold [4,5], and the incidence of acute respiratory infections, including influenza, decreased during the COVID-19 pandemic [5]. This could be attributed to infection control measures implemented during the pandemic, such as mandatory face masks and social distancing.

However, it is imperative to prevent influenza infection among the vulnerable population since the incidence of influenza infection began a resurgence similar to the levels before the COVID-19 pandemic in the second half of 2022 with the increased in-person interactions because of relaxed infection control measures [4,5]. Therefore, target population-specific flu vaccine promotions and stricter management to ensure safe vaccinations are needed to encourage flu vaccination. Furthermore, increased surveillance of adverse reactions following co-administration of flu and COVID-19 vaccines is required to prepare for the national influenza vaccination campaign in the 2023–2024 season.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

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

Author Contributions: Conceptualization: SHA, SLK. Data curation: SHA, MJS, SSJ, JYL, HAB, JEL. Methodology: SHA, MJS, SSJ, HAB. Supervision: SLK, HML, JHP. Writing–original draft: SHA, HAB, SLK. Writing–review & editing: SHA, SLK, HML.

Supplementary data are available online.