Graphical abstract

Introduction

In the Republic of Korea, primary doses of the Pfizer-BioNTech (BNT162b2) coronavirus disease 2019 (COVID-19) vaccination series initially became available for adolescents aged 16 to 17 years on October 18, 2021, and for those aged 12 to 15 years on November 1, 2021, with the aim of reducing the risk of infection and transmission [1]. Starting on March 14, 2022, monovalent booster vaccinations using the BNT162b2 COVID-19 vaccine were distributed to adolescents aged 12 to 17 years who had completed the primary series (2 doses) of COVID-19 vaccination. The Ministry of Food and Drug Safety granted import approval for the Omicron BA.1-containing bivalent BNT162b2 COVID-19 vaccine (hereafter referred to as Pfizer BA.1 vaccine) on October 7, 2022 [2] and issued an emergency use authorization for the Omicron BA.4 and BA.5-containing bivalent BNT162b2 COVID-19 vaccine (hereafter referred to as Pfizer BA.4/5 vaccine) for use in adolescents aged ≥12 years on October 17, 2022 [3]. The Pfizer BA.1 vaccine (30 µg) demonstrated higher immunogenicity against the Omicron variant and similar reactogenicity to the prototype BNT162b2 vaccine (30 µg) in clinical data [4,5]. In addition, the Pfizer BA.4/5 vaccine (30 µg) exhibited 2.6 times higher neutralizing antibody responses than the monovalent vaccine in preclinical data [5]. The bivalent COVID-19 booster vaccination, using Pfizer BA.1 and BA.4/5 vaccines, initially commenced on November 7 and November 14, 2022, respectively, for adults aged ≥18 years who had completed the primary series of COVID-19 vaccinations and was expanded to include adolescents aged 12 to 17 years on December 12, 2022 [6].

Despite the widespread availability of bivalent COVID-19 booster vaccines for adolescents aged 12 to 17 years in the Republic of Korea, vaccination coverage for the bivalent booster dose remained relatively low (15.1%) as of June 10, 2023 [7]. The low coverage can be attributed to reduced interest in COVID-19 vaccination, stemming from the need for multiple doses due to waning immunity over time, along with the decreasing severity of COVID-19 and prior instances of COVID-19 infection. Moreover, insufficient experience and information regarding the safety of the bivalent vaccine have contributed to the lower vaccination coverage. Studies suggest that COVID-19 vaccine hesitancy is primarily motivated by safety concerns [8,9]. The World Health Organization (WHO) identified vaccine hesitancy as one of the top 10 threats to global health in 2019 [10], and the absence of safety information is a significant contributor to vaccine hesitancy. Vaccine hesitancy was likely exacerbated by the reports of myocarditis and pericarditis among young adults after mRNA-based COVID-19 vaccinations. The WHO defined acute cardiovascular injury including myocarditis following COVID-19 vaccination as an adverse event of special interest (AESI). Potential causality with the COVID-19 vaccine required confirmation by further studies [11]. Therefore, studies like those done in the United States (US) and Denmark have been conducted globally to compare incidence rates and analyze the risk factors for myocarditis and/or pericarditis [12,13]. In addition, updated lists of the potential associations between COVID-19 vaccines and myocarditis/pericarditis were issued in March and May 2022, respectively, by the COVID-19 Vaccine Safety Research Committee in the Republic of Korea [14]. Therefore, it is necessary to collect additional safety information on all vaccines in use, leveraging our experiences with each vaccine to build trust in vaccination. However, research on bivalent vaccines based on real-world data to provide safety information on the bivalent COVID-19 booster dose for adolescents aged 12 to 17 years is currently limited.

The Korea Disease Control and Prevention Agency (KDCA) operates the COVID-19 vaccination management system (CVMS) at the national level to detect adverse events following immunization (AEFIs) and collect information, regardless of the causation between vaccines and events [15]. Doctors and forensic pathologists report AEFIs for each dose of COVID-19 vaccination to the CVMS under the Infectious Disease Control and Prevention Act. Moreover, ongoing safety monitoring is being conducted through a text message survey (TMS) to investigate self-reported adverse events (AEs) following COVID-19 vaccination [16]. The present study analyzed the results of COVID-19 vaccine safety monitoring for AEs. This included the assessment of myocarditis and pericarditis cases following COVID-19 bivalent booster vaccination in adolescents aged 12 to 17 years in the Republic of Korea, comparing them with those following monovalent booster vaccination. This study also aimed to share the safety profile of COVID-19 vaccine bivalent booster doses.

Materials and Methods

Data Collection and Study Subjects

As of May 20, 2023, the number of booster doses administered to adolescents aged 12 to 17 years in the Republic of Korea was 365,126 for the monovalent first booster from March 14, 2022, and 40,628 for the bivalent first booster (Pfizer BA.1 or BA.4/5 vaccines) from December 12, 2022. Vaccinations other than the monovalent or bivalent first booster (e.g., 1 case of second booster/fourth dose) and vaccinations administered outside of the Republic of Korea or before authorization for use of the booster vaccination were excluded. This study analyzed AEs reported to 2 COVID-19 vaccine safety databases (the CVMS and TMS) (Figure 1).

COVID-19 Vaccination Management System

We used data on AEs reported to the CVMS for adolescents aged 12 to 17 years following monovalent or bivalent first booster doses. A total of 966 and 27 AEs were reported out of 365,126 and 40,628 booster doses of monovalent and bivalent vaccines, respectively (Figure 1). Vaccine recipients could report AEs multiple times following vaccination, with each report potentially including varied symptoms or signs. AEs were categorized as non-serious or serious events according to the Guidelines for AEs Following COVID-19 Immunization in the Republic of Korea [17]. Non-serious AEs included common reactions such as redness, pain, and swelling at the injection site; myalgia; fever; headache; chills; and others. Serious AEs included death; suspected anaphylaxis; AESIs such as myocarditis, pericarditis, or thrombocytopenia syndrome (TTS); intensive care unit (ICU) admission; life-threatening events; permanent disability or sequelae; and others. Another method of data collection for AEs was a text messaging-based survey, benchmarked from the v-safe after vaccination health checker in the US [18].

Text Message Survey

As new brands of COVID-19 vaccine were introduced, the KDCA conducted the TMS to monitor AEs among vaccine recipients for 7 consecutive days following the date of COVID-19 vaccination, based on informed consent [16]. Initially, text messages were sent to recruit 5,000 adolescents for monovalent vaccination and 3,000 for bivalent vaccination. However, due to declining response rates over time, the survey was closed prematurely. The response rates among adolescents aged 12 to 17 years who participated in the TMS was 2.3% (3,523 out of 152,706) after monovalent booster vaccination from March 21 to April 3, 2022, and 6.2% (545 out of 8,789) after bivalent booster vaccination from December 14, 2022, to January 14, 2023 (Figure 1). The survey inquired about experiences of local AEs (pain, redness, swelling, itching, and urticaria at the injection site) and systemic AEs (fever or heat, chills, headache, joint pain, myalgia, fatigue, nausea, vomiting, diarrhea, abdominal pain, rash, armpit tenderness, and chest pain), limits to normal daily activities, and visits to medical facilities (emergency rooms, hospital admissions, clinics) following vaccination. Respondents were able to report multiple AEs on each day after vaccination.

Eligibility and Causality Assessment

In the Republic of Korea, the causality assessment of AESIs is conducted by the Vaccine Injury Investigation Committee (VIIC) and the Vaccine Injury Compensation Committee (VICC) operated by the KDCA. The VIIC promptly conducted epidemiological investigations for cases of serious AEs (e.g., death and ICU admission) including AESIs, and reviewed the causal association between the events and COVID-19 vaccination. Reported cases of AESIs, including anaphylaxis, myocarditis, pericarditis, TTS, and idiopathic thrombocytopenic purpura were reviewed for diagnostic eligibility based on the modified Brighton collaboration definition, utilizing diagnostic flowcharts developed by the expert advisory groups [15,19]. Cases related to non-mRNA-based COVID-19 vaccination, occurring >42 days post-vaccination, or attributed to causes other than COVID-19 vaccination were excluded from the causality assessment. This study utilized confirmed cases of myocarditis and pericarditis that had completed the eligibility and causality assessment process for AEs following monovalent and bivalent booster doses in adolescents aged 12 to 17 years in the Republic of Korea.

Statistical Analysis

The frequencies and reporting rates per 100,000 administered doses for both non-serious and serious AEs reported to the CVMS were analyzed according to sex, age group, and vaccine type (monovalent and bivalent boosters). Symptoms and signs reported as AEs were described in decreasing order of reporting rates per 100,000 doses. AEs reported to the CVMS were suspected cases and did not indicate medically confirmed diagnoses. The frequencies of AEs and health conditions reported to the TMS were analyzed by vaccine type (monovalent and bivalent boosters), considering respondents who reported at least 1 event during days 0¬ to 7 following vaccination as 1 occurrence. Chi-square or Fisher exact tests were conducted as appropriate to compare differences between vaccine types. A p-value less than 0.05 indicated statistical significance. Duplicate AEs in individuals who reported to both the CVMS and the TMS were not excluded from the analysis. The incidence rates per 100,000 person-days for myocarditis/pericarditis after each booster vaccination were compared using odds ratios (ORs) with 95% confidence intervals (CIs). All analyses in this study were performed using SAS ver. 9.4 (SAS Institute).

Ethics Approval

Surveillance activity based on the CVMS was conducted by the KDCA in accordance with the Infectious Disease Control and Prevention Act in the Republic of Korea. The study was not subject to institutional review board approval. Studies based on the TMS were exempted from review by the Public Institutional Review Board designated by the Ministry of Health and Welfare (No: P01-202206-01-033; No. P01-203206-01-011).

Results

AEs Reported to the CVMS

Between March 14, 2022, and May 20, 2023, 966 AEs following COVID-19 monovalent vaccinations were reported in adolescents aged 12 to 17 years; 950 (98.3%) were non-serious and 16 (1.7%) were serious, including AESIs for both COVID-19 vaccines (Table 1). From December 12, 2022, to May 20, 2023, 27 AEs following COVID-19 bivalent vaccination were reported in adolescents aged 12 to 17 years; 25 (92.5%) were non-serious and 2 (7.5%) were serious, including 1 (3.7%) case of suspected anaphylaxis and 1 (3.7%) AESI for COVID-19 vaccines. Throughout the study period, a total of 405,754 doses were administered, resulting in reporting rates per 100,000 doses of 264.6 after the monovalent booster and 66.5 after the bivalent booster, indicating a lower AE reporting rate for the bivalent vaccination.

The reporting rate per 100,000 doses by sex was 266.5 for males and 262.2 for females after the monovalent booster vaccination; and 63.0 for males and 70.1 for females after the bivalent booster vaccination, showing lower AE reporting rates in both males and females following the bivalent vaccination. By age group, the reporting rate per 100,000 doses was higher in adolescents aged 16 to 17 years (monovalent, 317.0; bivalent, 78.9) than in those aged 12 to 15 years (monovalent, 221.7; bivalent, 55.5). In addition, both age groups showed a higher reporting rate after the monovalent booster than after the bivalent booster.

Regarding non-serious AEs, higher reporting rates per 100,000 doses for headache, myalgia, fever, chest pain, dizziness, chills, injection site pain, dyspnea, and lymphadenitis were observed in adolescents after the monovalent booster (p<0.05) (Table 2). Serious AEs were rarely reported, ranging from 1 to 2 cases, except for acute cardiovascular injury. The reporting rates per 100,000 doses for acute cardiovascular injury following monovalent and bivalent booster doses were 2.4 and 2.5, respectively, with no statistically significant difference observed (p=0.999).

Self-reported AEs Collected through the TMS

The total number of adolescents aged 12 to 17 years who responded to the TMS was 3,523 after the monovalent booster vaccination from March 21 to April 3, 2022, and 545 after the bivalent booster vaccination from December 14, 2022, to January 14, 2023 (Table 3). A rate of 60.2% reported local AEs after the monovalent vaccination, while 34.1% reported local AEs after the bivalent vaccination (p<0.001). The results showed that both local and systemic AEs were reported less frequently after the bivalent vaccination than the monovalent vaccination in adolescents aged 12 to 17 years. Furthermore, 29.5% reported limits to normal daily activities after the monovalent booster and 16.7% after the bivalent booster, showing a significant difference (p<0.001) (Figure 2; Table 3). However, reported visits to medical facilities were 3.8% for the monovalent booster and 2.4% for the bivalent booster; this difference was not statistically significant.

Confirmed Cases of Myocarditis and Pericarditis

After reviewing 10 cases of acute cardiovascular injury reported to the CVMS based on diagnostic eligibility and causality assessment by the VIIC and VICC, 4 were confirmed as myocarditis, 1 as pericarditis, and 1 as myopericarditis. Four cases were deemed ineligible and excluded from the analysis (Table 4). Among the 6 cases of myocarditis/pericarditis, 5 were associated with the monovalent booster, while 1 was associated with the bivalent booster. Symptoms of myocarditis/pericarditis included chest pain or chest compression (5 reports), palpitations (2 reports), and dyspnea (1 report). Other symptoms included dizziness or syncope (1 report), intermittent fever (1 report), and cough (1 report). The causality assessment indicated that all confirmed cases were either probably or definitely related to vaccination. The incidence rates per 100,000 person-days for confirmed myocarditis/pericarditis following monovalent and bivalent booster doses were 0.03 and 0.05, respectively, with a statistically nonsignificant increased risk after the bivalent boosters (OR, 1.797; 95% CI, 0.210–15.386).

Discussion

The majority of AEs reported to the CVMS in adolescents aged 12 to 17 years following both monovalent and bivalent booster doses of COVID-19 vaccination were non-serious, reported at 98.3% for the monovalent booster and 92.5% for the bivalent booster. A greater proportion of non-serious AE reports was consistently observed in the US; however, a higher proportion was reported after the bivalent booster (94.3%) than the monovalent booster (91.6%) [20,21]. The variance in reporting patterns may be affected by shifts in safety concerns over time within the Republic of Korea. For instance, the initiation of monovalent booster vaccinations for 12- to 17-year-olds in March 2022 coincided with the release of an updated list associating COVID-19 vaccines with myocarditis (March 2022) and pericarditis (May 2022) in the Republic of Korea [14]. This likely stimulated safety concerns among the vaccine recipients or their parents, leading to an increase in reporting symptoms related to vaccine reactogenicity such as headache, fever, and myalgia following the monovalent boosters. Therefore, a higher proportion of non-serious AEs following monovalent boosters may have been observed in the Republic of Korea than after bivalent boosters. A previous study on the impact of media coverage on influenza vaccination in the Republic of Korea found a sharp increase in reporting of AEs during periods when death-related issues were highlighted in the media [22]. In addition, when comparing the data on AEs between the primary series of COVID-19 vaccinations and the booster doses in 12- to 17-year-olds in the Republic of Korea, no major difference in the reporting rates was observed for the monovalent booster. However, the reporting rates for AEs per 100,000 vaccinations were 4 times lower for the bivalent booster (66.5) than for the primary doses (dose 1, 268.1; dose 2, 295.8); furthermore, the vast majority of AEs were non-serious, reflecting the safety of the bivalent booster doses [1].

Among the 6 cases of myocarditis/pericarditis according to diagnostic eligibility, 5 were associated with the monovalent booster, while 1 was associated with the bivalent booster. The difference between the 2 incidence rates was not statistically significant, with higher risk after the bivalent boosters than the monovalent boosters (OR, 1.797; 95% CI, 0.210–15.386). Conversely, studies in Israel and the US identified a lower risk of myocarditis, pericarditis, or acute cardiovascular injury following the bivalent booster when compared to the monovalent booster [23,24]. Our study identified 6 cases of myocarditis/pericarditis in adolescents aged 12 to 17 years after the booster doses, and the majority did not progress to severe conditions and recovered within a few days after treatment (Table 4). In other studies, most patients who developed myocarditis after COVID-19 mRNA vaccination reportedly recovered within a few days [25]. Considering the effectiveness of the bivalent mRNA COVID-19 vaccines in children and adolescents against severe acute respiratory syndrome coronavirus 2 infection and symptomatic COVID-19 [26], this study supports that the benefits of COVID-19 vaccination, including prevention of severe outcomes and death, outweigh the potential risks [24].

In this study, the proportion of local AEs in 12- to 17-year-olds, including pain and swelling, reported through the TMS was lower after the bivalent vaccination than after the monovalent vaccination (monovalent: pain 55.9%, swelling 18.8%; bivalent: pain 31.9%, swelling 3.1%). This aligns with results observed in the US, where lower reporting rates for pain and swelling were noted following bivalent vaccination (monovalent: pain 80.0%, swelling 18.8%; bivalent: pain 66.9%, swelling 13.7%) [20,21]. The most frequent systemic AEs reported to the TMS for 12- to 17-year-olds following booster doses were headache (monovalent, 41.0%; bivalent, 23.3%), fatigue (monovalent, 38.8%; bivalent, 21.7%), and myalgia (monovalent, 37.6%; bivalent, 21.5%), with a lower proportion after bivalent vaccination. These results are consistent with the findings reported for adolescents aged 12 to 17 years in the US (monovalent: headache 55.9%, fatigue 58.5%, myalgia 46.2%; bivalent: headache 36.3%, fatigue 45.2%, myalgia 33.6%) [20,21].

Based on a review of several studies, the majority of AEs following both monovalent and bivalent vaccination were found to be non-serious. In a study by Andersson et al. [27], which included adults aged ≥50 years in Denmark, the use of bivalent mRNA COVID-19 booster vaccines was not associated with an increased risk of 27 types of AE. Similarly, Yamin et al. [23] found no elevated risk of non-COVID-19 hospitalization after monovalent and bivalent booster doses in Israel. Studies conducted in the Republic of Korea also supported the favorable safety profile and effectiveness of bivalent vaccines. Hyun et al. [28] reported lower estimated reactogenicities after the bivalent vaccine than the monovalent vaccine, and Chae et al. [29] identified better effectiveness of bivalent vaccines against COVID-19 than monovalent vaccines in adults in the Republic of Korea. Despite the findings from those studies, vaccination coverage with the bivalent booster dose in adolescents aged 12 to 17 years in the Republic of Korea remained low (15.1%) as of June 10, 2023 [7]. A recent Korean study identified that concerns about adverse reactions were the most significant contributor to COVID-19 vaccine hesitancy among parents of 7- to 18-year-old children [30]. Other countries including Israel, China, the US, and Japan also indicated that concerns over vaccine safety and side effects were the major cause of parental hesitancy in vaccinating their children against COVID-19 [31–35]. In addition, misinformation and conspiracy theories about vaccines from unidentified sources on the internet and social media have contributed to parental COVID-19 vaccine hesitancy [33–37]. While parents may hesitate to vaccinate their children due to concerns about AEs, our study findings indicate that AEFIs are mostly mild symptoms, and serious AEs are extremely rare, with the majority recovering within a few days. In this respect, health authorities are required to take the lead in providing accurate information on COVID-19 vaccines and their AEs to increase vaccine uptake [31,38].

This study had a few limitations. First, since the data reported through the CVMS were solely from individuals visiting medical facilities with symptoms, the results might be subject to underreporting. Second, as the TMS targeted adolescents aged 12 to 17 years and was based on self-reports by vaccine recipients or their guardians and parents, the reported AEs did not indicate medically confirmed diagnoses. Third, the results cannot be generalized to the entire population of vaccine recipients in the Republic of Korea because the text messages were sent during the initial period after the introduction of each monovalent and bivalent booster vaccination. Fourth, not all AEs reported to the CVMS and TMS were assessed for causality between the event and the vaccines. Last, individuals with a history of severe allergies to mRNA-based vaccine components [6], or those who did not wish to receive mRNA-based vaccines due to suggestive symptoms of myocarditis/pericarditis (e.g., chest pain, dyspnea, or palpitations) may have opted for substitution with a recombinant protein-based vaccine (i.e., NVX-CoV2373). In cases where individuals received non-mRNA-based vaccines or did not receive a vaccine, reporting biases may have occurred. Nonetheless, to the best of our knowledge, this is the first study to use analysis of national vaccine safety surveillance data to compare AEs following the monovalent and bivalent COVID-19 booster vaccination in adolescents aged 12 to 17 years in the Republic of Korea. This study identified that AEs in 12- to 17-year-olds following the bivalent booster were less frequent than those following the monovalent booster. Therefore, we believe that this study provides crucial information on the vaccine safety profile for adolescents aged 12 to 17 years in the Republic of Korea, underscoring the importance of gathering research data to inform future vaccine administration practices via ongoing safety monitoring. Further research is necessary to comprehensively assess the long-term safety of COVID-19 vaccines, particularly by analyzing data over time to identify potential rare AEs that may emerge in the future. In addition, providing ongoing guidance to healthcare workers and the general public on reporting AEFIs and simplifying the reporting system are essential for reducing the likelihood of missing AEs and enhancing overall vaccine safety. According to the recently announced COVID-19 vaccination plan, the bivalent vaccine is no longer available. However, it is possible that an updated vaccine could be developed in the future based on the circulation of new COVID-19 variants, similar to influenza vaccines that vary each year following the WHO recommendations for virus strains, leading to the production of trivalent or quadrivalent vaccines. We hope that the results of this study on the distribution and safety of the COVID-19 bivalent vaccine will serve as a significant reference.

HIGHLIGHTS

• Despite the widespread availability of bivalent mRNA COVID-19 boosters for adolescents aged 12 to 17 years in the Republic of Korea, vaccination coverage has remained low and real-world data to provide safety information are currently limited.

• This study aimed to provide safety information regarding bivalent mRNA COVID-19 boosters by analyzing the results of monitoring COVID-19 vaccinations for adverse events in the Republic of Korea.

• Adverse events in 12- to 17-year-olds following the bivalent booster were less frequent than those following the monovalent booster in the Republic of Korea, emphasizing the need to collect research data through ongoing safety monitoring to guide future vaccine administration.

Article information

Ethics Approval

The surveillance activity based on the CVMS was conducted by the KDCA in accordance with the Infectious Disease Control and Prevention Act in the Republic of Korea; the study was not subject to institutional review board approval. The studies based on the TMS were exempted from review by the Public Institutional Review Board designated by the Ministry of Health and Welfare (No: P01-202206-01-033; No. P01-203206-01-011).

Conflicts of Interest

The authors have no conflicts of interest to declare.

Funding

None.

Availability of Data

The data used in this study are protected under the Personal Information Protection Act.

Authors’ Contributions

Conceptualization: MK, YK, YKL; Data curation: MK, SK, SHS; Formal analysis: MK, SK, SKC; Investigation: SK, SKC, SHS; Methodology: MK, SK, YK; Supervision: YK; Validation: MK, YK, YKL; Visualization: MK, SK; Writing–original draft: MK, SK, YK; Writing–review & editing: all authors. All authors read and approved the final manuscript.

Figure 1.

Flowchart for data collection and study subjects.

COVID-19, coronavirus disease 2019; CVMS, COVID-19 vaccination management system; TMS, text message survey. ^a)Monovalent booster vaccination from March 14, 2022, to May 20, 2023. ^b)Bivalent booster vaccination from December 12, 2022, to May 20, 2023. ^c)Monovalent booster vaccination from March 21 to April 3, 2022. ^d)Bivalent booster vaccination from December 14, 2022, to January 14, 2023.

Figure 2.

Adverse events and health conditions self-reported through the text message survey by adolescents aged 12 to 17 years who received the Pfizer-BioNTech coronavirus disease 2019 vaccine first booster doses according to vaccine type: monovalent booster vaccination from March 21 to April 3, 2022; bivalent booster vaccination from December 14, 2022, to January 14, 2023 (Republic of Korea). Values represent the percentage of respondents who reported adverse events and health conditions at least once during days 0 to 7 post-vaccination.

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