Assessing the determinants of influenza and COVID-19 vaccine co-administration decisions in the elderly

ABSTRACT

This research examines the low rate of co-administration of influenza and COVID-19 vaccines among seniors aged 65 and older in Korea, despite recommendations from authorities and academia worldwide. The study aimed to understand the influence of general characteristics and health beliefs on the vaccination choices of seniors, who were categorized into four groups based on their vaccination status: influenza only, COVID-19 only, both, or neither. A total of 400 participants, aged 65 and above, were selected through proportional stratified random sampling from five major Korean regions for a survey conducted between November 24th and December 15th, 2023. The results indicated no significant differences in general characteristics across these groups. However, regarding the health beliefs showed significant differences in perceived susceptibility and self-efficacy between the influenza-only and co-administration groups. Higher levels of perceived susceptibility and self-efficacy were associated with choosing co-administration. Contrary to previous studies focusing on safety concerns as a primary factor in vaccine hesitancy, this study highlights the role of individual health-related beliefs, particularly perceived susceptibility and self-efficacy, as critical in influencing the decision for co-administration among the elderly in Korea.

KEYWORDS:

• Co-administration
• vaccines
• COVID-19
• influenza
• elderly

Introduction

As the world continues to grapple with the ongoing challenges posed by COVID-19 and influenza, the public health significance of these infectious diseases remains at the forefront of global health concerns. The lifting of the Public Health Emergency of International Concern (PHEIC) status by the World Health Organization (WHO) does not diminish the ongoing impact of COVID-19 on global health.¹ Despite this change, in 2022, the Republic of Korea continued to experience the substantial effects of the pandemic, with COVID-19 ranking as the third leading cause of death across all age groups in the country.² Following the relaxation of COVID-19 mandates, such as mask-wearing and social distancing, there has been a resurgence of influenza, which was previously suppressed during the peak periods of the COVID-19 pandemic. This resurgence has been particularly noticeable since the 2022–2023 season, marking a return to the traditional seasonal epidemic pattern of influenza.³ Interestingly, in the Republic of Korea, influenza has maintained a year-round circulation in 2023, deviating from its usual seasonal pattern.⁴ This change underscores the dynamic nature of viral infections and the importance of adaptable public health strategies.

Vaccination is paramount in preventing pandemics like COVID-19 and influenza.^2,5 Despite the global availability of vaccines, there is an emerging trend of vaccine refusal or hesitancy.⁶ The decline in vaccination rates could lead to serious public health issues, including the escalation of COVID-19 and influenza pandemics and an increase in mortality.⁷ Therefore, urgent public health strategies to address vaccine hesitancy are imperative. The co-administration of COVID-19 and influenza vaccines has emerged as a potential approach to enhance immunization coverage against the co-circulation of these two viruses. Recent studies have shown that simultaneously administering the COVID-19 and seasonal influenza vaccines to diverse population groups does not compromise the immune response.^5,7–10 Furthermore, this co-administration approach does not elevate safety concerns compared to administering each vaccine individually.^7,9

Evidence related to efficacy and safety supports the concurrent vaccination approach, which has been proven effective in increasing overall vaccination coverage. This is particularly important in the context of the ongoing co-circulation of COVID-19 and seasonal influenza. These findings underscore the potential of co-administration as a practical and effective public health strategy. Administering both COVID-19 and influenza vaccines together may increase overall vaccination coverage. Offering both vaccines in a single visit not only reduces the burden on individuals to make multiple healthcare appointments but also addresses vaccine hesitancy by simplifying the process.⁸

WHO, European Union, United States of America, United Kingdom, and Japan promoted the concomitant administration of influenza and COVID-19 vaccines, especially for the vulnerable population.⁹ The elderly population is particularly vulnerable COVID-19 and influenza, making their protection a public health priority.⁵ Older adults often have weaker immune systems and are more likely to have underlying health conditions that can exacerbate the effects of these respiratory viruses. This susceptibility leads to a higher risk of severe illness, hospitalizations, and even death.¹⁰ Similarly, influenza is known to be especially dangerous for older adults. Each year, a significant proportion of influenza-related hospitalizations and deaths occur in individuals aged 65 and older.¹¹ Despite active encouragement for vaccination among the elderly populations worldwide, the vaccination rates for COVID-19¹² and Influenza¹³ have been shown to be low.

Therefore, co-administering the COVID-19 and influenza vaccines can be particularly beneficial for the elderly, as it simplifies the process of receiving necessary immunizations and provides double protection. Despite the acknowledged importance, the vaccination rates for influenza and COVID-19 among individuals aged 65 and over demonstrated a significant disparity in the 2022–2023 season. While the vaccination rate for the 2022–2023 influenza season in the Republic of Korea, which commenced in a similar timeframe, was approximately 81.9% (aged 65 and over), the rate for COVID-19 was only in the 34.8% (aged 60 and over).¹⁴ Previous study has indicated that individuals vaccinated against influenza are over eight times likely to express a willingness to receive the COVID-19 vaccine compared to those not vaccinated for influenza, suggesting that a strategy of concurrent vaccinations could potentially enhance the uptake of COVID-19 vaccinations.¹⁵ Along with this reason, in the 2023–2024 season, the Korea Disease Control and Prevention Agency (KDCA) strongly recommended co-administration of both vaccines especially for the persons aged 65 and older.

The factors influencing vaccination decisions, as highlighted in the previous studies, integrate both the principles of the Health Belief Model (HBM) and various socio-demographic factors. The HBM is a critical psychological framework to understand health behaviors including vaccination.¹⁶ In the context of vaccination, an individual’s perception of their susceptibility to diseases like COVID-19 and influenza plays a crucial role. If they consider themselves at high risk, they are more likely to opt for vaccination. Similarly, perceived severity, or how serious they believe the disease to be, can motivate vaccination. The benefits of getting vaccinated, such as protection against severe illness, are weighed against perceived barriers, including concerns about vaccine safety, side effects, or access to vaccination services. Cues to action, such as public health campaigns or advice from healthcare providers, can also prompt individuals to get vaccinated. Additionally, self-efficacy, or an individual’s confidence in their ability to get vaccinated, is an essential factor. People who believe they can easily access and receive vaccines are more likely to follow through.¹⁷ Socio-demographic factors also have a significant impact on vaccination decisions. Age, gender, income, and education level are well known critical factors for the vaccination decision.¹⁸ However, little research has been conducted regarding the factors of the co-administration of COVID-19 and Influenza vaccines targeted at the elderly. Therefore, this research will fill the existing gap in the body of knowledge by leveraging a targeted vaccination strategy for the most vulnerable people. This approach enabled a comprehensive analysis of the factors influencing vaccination decisions across different segments of the population, providing valuable insights into public health strategies for improving vaccine uptake.

Materials and methods

Data sampling

This research is a cross-sectional study with structured questionnaires. The population consists of individuals aged 65 and older residing in the five major regions of South Korea, including Seoul, Gyeonggi/Incheon, Chungcheong, Honam, and Yeongnam. To describe the characteristics of the population according to generalization, we first calculated the sample size using the G*power 3.1 program. For logistic regression analysis, with a two-tailed test, odds ratio of 1.6, significance level of .05, and power of .95, a sample size of 377 was calculated. Considering a dropout rate of 10%, a total of 400 participants were set as the study population. Participants were selected through proportional stratified random sampling. The number of participants in each group was determined to ensure the representativeness of the sample, considering the ratio of data from the Immunization Registry Information System (IRIS)¹⁹, including groups vaccinated with the influenza vaccine, the COVID-19 vaccine, both vaccines, the co-administration group, and the unvaccinated group, totaling five groups. The survey period spanned from November 24th to December 15th, 2023. Given the advanced age of the participants, which could impede their ability to engage with an online survey effectively, the study opted for conducting the survey through one-on-one, face-to-face interviews. These interviews were administered by trained professionals from Southern Post, Inc., a specialized research company based in Seoul, Korea, to ensure accurate and reliable data collection.

Questionnaires

This study referred to previous research based on the HBM²⁰ and consisted of 21 items on seven subscales related to perceived susceptibility, perceived severity, perceived benefit, perceived barriers, cue to action, self-efficacy, and health behavior as shown in Appendix. Responses to the items were rated on a five-point Likert scale. Cronbach’s α was .77 in a previous study²⁰ and .65 in this study. In order to determine whether the questions in this questionnaire measure the intended concept, the Content validity index and Face validity were measured by five nursing professors and consisted of questions that were 0.9 or higher. Additionally, sociodemographic factors such as gender, income, and education level were also included. The primary dependent variable in this study was the co-administration of vaccines, specifically defined as receiving both the COVID-19 and influenza vaccines on the same day. To assess this, participants were asked whether they were vaccinated from September 2023 onwards, coinciding with the start of the 2023 influenza vaccination season. Independent variables were derived from responses to questions about perceived health beliefs and sociodemographic information such as gender, age, income, and comorbidity.

Statistical analysis

The analysis began with descriptive statistics to assess coverage rate of vaccination groups using data derived from the IRIS for January 4 in 2024. Then, to assess differences across vaccination groups this study used chi-square, ANOVA, and T-tests. These steps were crucial for identifying significant variations in independent variables, including health belief-related factors and sociodemographic aspects like age, gender, and income. The main analysis utilized logistic regression, chosen for its suitability in examining the binary dependent variable (vaccination co-administration vs. vaccination only for influenza), thus enabling a detailed understanding of the factors influencing the concurrent uptake of COVID-19 and influenza vaccines. The threshold for determining statistical significance was established at p < .05. Statistical analyses were conducted using SPSS for Windows, version 21.0 (IBM Corp., Armonk, NY, USA).

Results

COVID-19 and influenza vaccination rate in 2023

As of December 2023, in Korea,¹⁹ among the seniors aged 65 and older 0.6% received only the COVID-19 vaccine, 42.4% received only the influenza vaccine, 39.7% received both vaccines, and 10.1% received both vaccines simultaneously. 17.2% did not receive any vaccination as shown in Table 1, extracted from the IRIS. The participants were categorized into different groups: those who received the influenza vaccine (n = 120), the COVID-19 vaccine (n = 40), both vaccines but not simultaneously (n = 72), both vaccines on the same day (n = 48), and neither vaccine (n = 120).

Table 1. COVID-19 and influenza vaccination rate (N = 8,086,682*).

	COVID-19 only vaccination	Influenza only vaccination	All vaccinated^a	Co-administration^b	Unvaccinated
n	58,240	4,147,798	3,880,644	987,849	1,685,295
%	0.6%	42.4%	39.7%	10.1%	17.2%

*This number represents the population aged 65 and older in the Republic of Korea. It is derived from the Immunization Registry Information System, which is linked to the resident registration database of the Ministry of the Interior and Safety in the Republic of Korea.

^aReceiving the COVID-19 vaccine and influenza vaccine.

^bReceiving the COVID-19 vaccine and influenza vaccine on the same day.

Differences in health beliefs between the influenza vaccination group and the co-administration group

Table 2 shows the general characteristics of the subjects who participated in this study. No statistically significant differences were identified among groups according to general characteristics as shown in Table 3. The analysis revealed no significant differences in general characteristics such as gender, age, education, income, and disease history across these groups.

Table 2. General characteristics (N = 400).

	Content	n	%
Gender	Male	197	49.3
	Female	203	50.8
Age	65–69	140	35.0
	70–74	131	32.8
	≥75	129	32.3
Education	≤Elementary school	54	13.5
	Middle school	133	33.3
	High school	195	48.8
	≥University	18	4.5
Marital Status	Single	1	0.3
	Married	331	82.8
	Divorced	4	1.0
	Widowed	64	16.0
Monthly income	≤1 million won*	135	33.8
	1–2 million won	144	36.0
	2–3 million won	75	18.8
	3–4 million won	31	7.8
	4–5 million won	11	2.8
	5–6 million won	2	0.5
	≥7 million won	2	0.5
Occupation	Private Business	91	22.7
	Simple labor	33	8.3
	Agriculture	28	7
	Unemployed	248	62
Disease history	Yes	214	53.5
	No	186	46.5
Disease type (multiple responses) n = 214	Endocrine disease	64
	Cardiovascular disease	148
	Hyperlipidemia	32
	Musculoskeletal disease	27
	Gastointestinal disease	6
	Pulmonary disease	3

*1 million won ≒ 750 US dollar.

Table 3. Differences between groups according to general characteristics (N = 400).

	Content	COVID19	Influenza	Co-administration	All vaccinated	Unvaccinated	χ2	p
		n = 40	n = 120	n = 48	n = 72	n = 120
Gender	Male	22	52	28	36	59	3.81	.432
	Female	18	68	20	36	61
Age	65~69	20	40	14	26	40	5.15	.741
	70~74	11	40	17	23	40
	≥75	9	40	17	23	40
Education	≤Elementary school	6	17	5	7	19	11.97	.448
	Middle school	13	42	22	22	34
	High school	20	53	18	42	62
	≥University	1	8	3	1	5
Marital Status	Single	0	1	0	0	0	13.00	.369
	Married	33	102	39	56	101
	Divorce	0	1	0	3	0
	Bereavement	7	16	9	13	19
Average monthly income	≤1 million won	15	42	12	25	41	18.85	.760
	1–2 million won	10	43	19	30	42
	2–3 million won	11	24	8	10	22
	3–4 million won	3	7	6	3	12
	4–5 million won	1	2	2	4	2
	5–6 million won	0	1	0	0	1
	≥7 million won	0	1	1	0	0
Occupation	Yes	17	43	19	26	47	3.26	.515
	No	23	77	29	46	73
Disease history	Yes	22	60	29	43	60	3.26	.515
	No	18	60	19	29	60
Surgical history	Yes	5	21	6	11	20	1.06	.900
	No	35	99	42	61	100

Co-administration predictive factors

This study analyzed the differences in health beliefs between the influenza-only vaccination group and the influenza-COVID-19 co-administration group.^15,21 Significant differences were observed in perceived susceptibility (p = .003), severity (p = .040), and self-efficacy (p = .007) as shown in Table 4. Logistic regression analysis revealed that higher perceived susceptibility (OR = 1.466, 95%CI: 1.045–2.057) and self-efficacy (OR = 1.473, 95%CI: 1.112–1.953) increased the likelihood of co-administering influenza and COVID-19 vaccines as shown in Table 5.

Table 4. Differences in health beliefs between the influenza vaccination group and the co-administration group.

	Influenza only vaccination (N = 120)	Co-administration (N = 48)	t	p
Susceptibility	7.12 ± 1.361	7.81 ± 1.249	−3.063	.003**
Severity	10.66 ± 1.606	11.21 ± 1.414	−2.072	.040*
Benefits	16.12 ± 1.834	16.44 ± 2.082	−.985	.326
Barriers	8.59 ± 1.350	8.77 ± 1.477	−.756	.451
Cues to action	12.15 ± 1.470	12.46 ± 1.738	−1.164	.246
Self-efficacy	11.08 ± 1.607	11.90 ± 1.777	79.393	.007**
Health behavior	10.79 ± 1.847	10.94 ± 1.895	−.459	.647

*p < .05.

**p < .01.

Table 5. Co-administration predictive factors.

	p	Odds ratio	95% C.I.
			lower limit	maximum
Susceptibility	.027*	1.466	1.045	2.057
Severity	.988	1.002	.757	1.326
Benefits	.654	.951	.762	1.186
Barriers	.371	1.130	.865	1.478
Cues to action	.510	.906	.675	1.215
Self-efficacy	.007**	1.473	1.112	1.953
Health behavior	.352	.908	.740	1.113
	.014	.002

*p < .05.

**p < .01.

Discussion

While numerous countries are currently endorsing the simultaneous administration of influenza and COVID-19 vaccines to enhance the practicality of vaccination programs,²² there is a global trend where the rates of concurrent vaccination for COVID-19 or COVID-19 vaccination alone are lower compared to influenza vaccination.^23–25 There is also a growing tendency for increased anxiety and hesitation toward combined vaccination compared to individual vaccinations.²⁶

Consequently, the objective of this study was to identify factors that could bridge this gap. The findings indicate that the uptake of either the COVID-19 vaccine alone or its co-administration with the influenza vaccine was less than that of the influenza vaccine alone. The Korean government’s strong recommendation of the co-administration of COVID-19 and influenza vaccines in 2023 presented a challenge for the elderly population in adapting to this new vaccination strategy.

This investigation revealed that variations in general characteristics between different groups were not substantial, leading to an examination of the determinants of vaccination as per the HBM in addition to socioeconomic factors in the elderly. The study determined that perceived susceptibility and self-efficacy were significant influencing factors, echoing findings from earlier research.²⁷ Perceived susceptibility, the degree to which an individual feels at risk of contracting a disease, differs greatly among individuals.²⁸ Following the revocation of the PHEIC status by the WHO, there has been a public perception that COVID-19 is no longer a threat. However, it continues to be the predominant respiratory virus, contributing to an ongoing epidemic with a significant disease burden. This underscores the necessity of disseminating accurate information about the virus’s circulation and the current epidemic to the public.

Self-efficacy, the belief in one’s capacity to manage and execute the actions needed to handle a situation effectively,²⁹ is a crucial predictor of co-administration. This suggests that the perceived efficacy of the COVID-19 vaccine plays a pivotal role in an individual’s decision to vaccinate. There is an emerging trend where the significance of the COVID-19 vaccination is underestimated due to beliefs about its inability to provide complete protection against infection.³⁰ Nonetheless, the importance of the COVID-19 vaccine remains substantial. The COVID-19 virus continues to mutate, leading to variants that can evade immunity provided by previous vaccines.^31,32 As immunity wanes over time, there is an increasing necessity for new vaccines, specifically tailored to these emerging variants.^33,34 Periodic vaccination with these updated vaccines is essential to maintain effective protection against COVID-19.²⁴

Post-COVID-19 pandemic studies have frequently pointed to vaccine safety concerns as a primary factor in vaccination hesitancy.^{29,30,35–37} Interestingly, when comparing this factor with individuals who received either the influenza vaccine alone or the co-administration of influenza and COVID-19 vaccines, the results differed. The perceived barriers, including concerns about adverse reactions and safety issues, were not significant in these groups. This could be attributed to the fact that these groups had already been vaccinated, making them less sensitive to vaccine-related risks compared to those who had not received any vaccination. This suggests that rather than focusing solely on disseminating information about vaccine safety, emphasizing the ongoing risk of COVID-19 infection and the efficacy of new vaccines in providing protection might be more crucial. Such an approach could potentially enhance the rate of co-administration of vaccines.

A limitation of this study is that it surveyed the subjective health beliefs of 400 participants in a cross-sectional manner, which may hinder generalizing the findings to the entire elderly population of the Republic of Korea. Future large-scale longitudinal studies are necessary to ascertain whether the determinants of influenza and COVID-19 vaccine co-administration identified here are broadly applicable. However, this research is pioneering in analyzing the determinants of vaccination decisions following the introduction of recommendations for simultaneous vaccination. Thus, it provides valuable evidence for determining effective interventions to enhance future vaccination rates.

Author contributions

Conceptualization, SLK and HJ; methodology, SLK, SYK and SHB; validation, SLK, HML and HJ; formal analysis, SLK, MSL, SHB and HJ; writing – original draft preparation, SLK, SYK and HJ; writing – review and editing, SLK, SYK, MS, HML, SHB, MSL and HJ; supervision, SLK; project administration, SLK; funding acquisition, MSL and SHB. All authors have read and agreed to the published version of the manuscript.

Ethical considerations

The research adhered to ethical protocols, having been exempted by the Institutional Review Board (IRB) of Youngsan University (YSUIRB-202311-HR-141-02). In line with ethical practices, informed consent was procured from all participants by the survey practitioners. This process involved clearly explaining the study’s aims, procedures, and the voluntary basis of participation, while guaranteeing confidentiality and anonymity for all respondents.

Acknowledgments

We would like to express our deep gratitude to all the participants who took part in the survey for this study. Additionally, during the preparation of this work the authors used AI-assisted tool in order to grammar check and language improvement. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

The data are available upon request. All requests were addressed by the corresponding author.

Additional information

Funding

This study received financial support from the National Immunization Program, which is administered by the Korea Disease Control and Prevention Agency. This study also received a research grant from Youngsan University.

Appendix.

Measurement of HBM

Table

Subscale	Items	Mean (SD)	Cronbach’s α
Perceived Susceptibility	I believe that if I do not receive a vaccination, I am at a higher risk of contracting infectious diseases.	3.70 (0.87)	.62
	I think I am particularly susceptible to infectious diseases compared to other people.	3.20 (1.01)
Perceived Severity	I feel that if I were infected with an infectious disease, I would experience severe pain or sequelae.	3.48 (0.90)	.58
	I believe that even if I were infected with an infectious disease, the chances of recovery would be very low.	3.19 (0.96)
	I believe that if I were to contract an infectious disease, it could lead to social and economic problems.	3.69 (0.74)
Perceived Benefits	I believe that vaccination is effective in preventing serious pain and complications caused by infectious diseases.	3.91 (0.66)	.74
	I believe that if I receive vaccination, the risk of infecting family members or people around me with the corresponding infectious disease decreases.	3.99 (0.74)
	I believe that if I receive vaccination, even if I contract an infectious disease, I can easily overcome it.	3.76 (0.86)
	I believe that if I receive vaccination, I can engage confidently in interpersonal relationships and social activities.	3.79 (0.69)
Perceived Barriers	I find it difficult to access the online vaccination reservation system.	3.90 (0.86)	.65
	It is inconvenient for me to go to the hospital for vaccination due to transportation issues.	2.42 (0.94)
	I do not want to undergo vaccination due to a strong distrust of it.	2.42 (0.91)
Cues to action	If the media (TV, newspapers) and doctors explain the benefits of vaccination, I am more likely to receive vaccination.	3.94 (0.67)	.76
	I trust and follow the national vaccination recommendations.	3.85 (0.80)
	If friends and family recommend vaccination, I am more likely to receive vaccination.	3.96 (0.74)
Self-efficacy	I am well-informed about the timing, methods, and vaccination procedures.	3.46 (0.84)	.79
	I am confident in my ability to understand and practice infection prevention guidelines.	3.67 (0.83)
	I can select and utilize accurate and reliable information about infection prevention.	3.59 (0.72)
Health behavior	I engage in regular physical exercise appropriate for my age.	3.40 (0.94)	.44
	I consume a healthy diet and supplements.	3.52 (0.85)
	I practice smoking cessation or moderation for my health.	3.80 (0.95)

SD: standard deviation.