Globally, infectious disease outbreaks have increased steadily over the last 30 years, contributing significantly to the global burden of disease [1
]. One such infectious disease is seasonal influenza. An estimate of 290,000–650,000 deaths occur annually around the world due to influenza-associated respiratory diseases [2
]. Another perennial and severe disease in tropical countries is dengue fever. Globally, an estimated 3.9 billion people are at risk of contracting the disease [3
], and Southeast Asia alone has an estimated economic burden of USD 950 million each year [4
As a city state hosting high volumes of international travelers [5
], Singapore’s population is continually at risk of exposure to imported infectious diseases. As such, it also has the added potential of becoming a hub for exporting infectious diseases to other places around the globe. Singapore, being close to the Equator, is also at risk of both the southern and northern strains of seasonal influenza, and susceptible to the virus all year round [6
]. The country is also at risk of dengue. Over the last few decades, the frequency and magnitude of dengue fever has increased in tandem with the increased density of Singapore’s urban population [7
]. In response, the Singapore government has implemented regular initiatives and campaigns for dengue prevention [8
Immunisation through the administration of vaccines has long been acknowledged as a cost-effective and crucial health intervention among the general population by building herd immunity [10
]. The World Health Organisation (WHO) [11
] has acknowledged that vaccinations prevent approximately 2–3 million deaths per year from a host of potentially dangerous but preventable diseases. To encourage vaccinations, WHO has formulated a Global Action Plan (GAP) which calls for a global increase in seasonal vaccine use to mitigate the burden of seasonal influenza worldwide, as well as to increase global preparedness levels against pandemics [12
Whilst vaccination is an effective preventive measure against influenza [14
], developed countries such as the United States, the United Kingdom, and Japan have low vaccination uptake rates, ranging from approximately 30–40% [15
]. In Singapore, influenza vaccinations of residents through general hospitals and private clinics are recommended, especially for high risk populations like the elderly and young children. Despite this ease of access, the uptake rate has been low at merely 15.2% in 2013 [17
]. Various reasons have been proposed to explain these low uptake rates. For example, studies have suggested that individuals who perceive themselves as being less susceptible to influenza and who have misconceptions on the potential severity of the disease tend to remain unvaccinated [16
]. In short, uptake of vaccinations across populations is generally low unless the perceived severity surrounding the disease is high [19
For dengue, the vaccine Dengvaxia, which was first distributed in 2017, has also been approved and made accessible in Singapore [20
]. However, due to factors relating to the recency of this vaccine, it has not been as well promoted as the influenza vaccination, and no data has yet been released on the uptake rate within Singapore. Singapore is more often placed on high alert with frequent news coverage on new dengue clusters [21
] and deaths due to dengue fever [22
]. This contrasts with the relatively quiet reporting on influenza. As such, the threat of dengue fever is often a more salient concern among the Singapore population.
If Singapore were able to achieve high rates of immunisation, the resulting herd immunity effect would make it more resilient against preventable pandemics. This would also deny infectious diseases the opportunity to use Singapore as a staging ground to infect other parts of the world. In light of the potential to prevent and deny infectious disease access through Singapore via high vaccination rates, it is imperative that we understand the drivers of vaccine uptake which can improve vaccination rates to a suitable level.
Hence, this research proposes to understand the factors that predict vaccine uptake intentions for two major diseases—influenza and dengue. The following research objectives are addressed:
Research objective 1: To compare the knowledge and perceptions of influenza and dengue and their respective vaccines.
Research objective 2: To understand the key drivers within each of the diseases in predicting vaccine uptake intentions.
This study attempts to make a within-subjects comparison of the drivers of vaccine uptake for influenza and dengue fever among Singaporeans and Singapore permanent residents. Both diseases are well-known and recur frequently and have been the subject of targeted health campaigns (Influenza [23
]; Dengue [8
]). Therefore, both diseases are comparably salient and the perceived knowledge about them is relatively high. However, because dengue fever causes deaths more directly in vulnerable populations than influenza, dengue fever is expected to be perceived as a more serious illness. This study further compares the Health Belief Model (HBM) constructs of susceptibility, perceived barriers, and perceived benefits as predictors of intention to vaccinate. To our knowledge, no study has attempted a within-subjects comparison of disease perception in predicting vaccination uptake.
Among the respondents recruited, the mean age was 43.41 (SD = 14.65), and 61.3% were females. Table 1
presents the demographic characteristics of the participants. Reflecting the Singapore population, the majority were of Chinese ethnicity (78.5%) and have at least completed General Certificate of Education (GCE), O (Ordinary)/N (Normal)/A (Advanced) level or Diploma (56.6%) (See Table 1
Only 23.7% of respondents reported that they would probably or definitely intend to vaccinate in the next year against influenza, while 21.0% said they would probably or definitely vaccinate against dengue. The majority of respondents were undecided as to whether they intend to get vaccinated against influenza and dengue (41% and 49.5% respectively).
Perceived susceptibility was low for both diseases. A mean score of 2.32 (SD = 0.90) and 2.22 (SD = 0.89) was found for influenza and dengue, respectively, with respondents perceiving themselves to be significantly more susceptible to influenza than dengue. The mean for perceived severity and benefits to getting vaccinated, on the other hand, were higher for both diseases. The mean scores for influenza and dengue were 3.18 (SD = 0.98) and 3.99 (SD = 0.87) for perceived severity, and 3.54 (SD = 0.94) and 3.69 (SD = 0.91) for perceived benefits, respectively. For both predictors, however, dengue was significantly scored higher than influenza. The perceived barriers to both influenza and dengue vaccine uptake were 3.07 (SD = 0.88) and 3.00 (SD = 0.86), respectively, and they were not significantly different. Lastly, in terms of knowledge of the disease, the participants perceived themselves as knowing significantly more about dengue (M = 3.17, SD = 0.84) than influenza (M = 2.87, SD = 0.87), although both scores were somewhat average (See Table 2
To understand how these factors affected the two diseases separately, multiple regression analyses were conducted to determine the relationship between the five predictors (severity, susceptibility, benefits, barriers, and knowledge) of vaccine uptake and the intention to get vaccinated in the following year for influenza and dengue. To fully assess the extent of influence of the independent variables, gender, age, ethnicity, and educational attainment were controlled for. Results show that the five predictors explained a significant proportion of the variance (R2
=16.7%). Specifically, intention to get vaccinated for influenza was significantly predicted by higher perceived susceptibility (β
= 0.25, t
= 7.93, p
< 0.001), and higher perceived benefits of getting vaccinated (β
= 0.31, t
= 8.95, p
≤ 0.001). However, perceived susceptibility of the disease, knowledge of the disease, and perceived barriers of getting vaccinated were not significant predictors of vaccine uptake (See Table 3
On the other hand, for dengue vaccine uptake, regression analyses show that the model explain 18.9% of the total variance. Specifically, higher perceived susceptibility (β
= 0.28, t
= 9.02, p
≤ 0.001), higher perceived benefits of getting vaccinated (β
= 0.32, t
= 9.29, p
≤ 0.001), and lower barriers to getting vaccinated (β
= −0.13, t
= −4.13, p ≤ 0.001) significantly predicted behavioural intention to vaccinate. Severity and knowledge of dengue were not found to significantly predict intention to vaccinate (See Table 4
This is one of the first studies conducted in Singapore that examines factors influencing the potential uptake of vaccines for two different diseases—influenza and dengue—using a within-subject design. Influenza and dengue are two major diseases that affect the Singapore population. Comparing factors such as susceptibility, severity, and knowledge of the diseases and benefits and barriers of the vaccines can help in understanding the way the public perceive these diseases and their vaccination uptake intentions. Overall, intention to get vaccinated was low for both diseases, with only approximately one fifth of the participants indicating that they would consider getting vaccinated against influenza and dengue the following year.
The study shows that perceived susceptibility to influenza and dengue were low, suggesting that participants do not consider themselves to be at high risk of getting the diseases. However, perceived susceptibility was significantly higher for influenza than dengue, indicating that participants think they are more likely to get influenza than dengue. One major reason could be because influenza is more prevalent in Singapore. Official sources stated that in 2017, thirteen people per day sought treatment for influenza from polyclinics [33
], while only ninety-one people have been diagnosed with dengue within the year [34
]. Also, this study found that approximately three times as many people had been diagnosed with influenza in the last year, in comparison to those who had ever been diagnosed with dengue. More people are aware of others who have been diagnosed with influenza and may therefore consider themselves more susceptible to it. In addition, since dengue is a vector borne disease, individuals could be taking active measures such as applying mosquito repellent sprays or avoiding high mosquito breeding areas in order to ensure their safety. At the same time, individuals may feel uncertain of how to protect themselves from influenza as they cannot predict how and when they will get it during social interactions.
Perceived severity had the highest score of all the predictors for dengue suggesting that dengue is seen to be a dangerous disease. Severity was also significantly higher for dengue than influenza, which was predominantly seen as only moderately severe. This could be because Singapore is more often placed on high alert about deaths due to dengue fever [22
]. This contrasts with the frequent news coverage depicting influenza as a mild disease. Perceived benefits of getting vaccinated were moderately high for both influenza and dengue, implying that getting vaccinated is perceived as effective in reducing the risks of getting the disease. In addition, getting vaccinated against dengue is seen as significantly more beneficial than getting vaccinated against influenza, potentially due to the higher perceived severity of dengue, which may suggest that more action is needed to prevent the disease.
Both barriers to getting vaccinated and perceived knowledge of the diseases had middling mean scores, indicating that the participants were neutral towards the tangible and psychological costs of getting vaccinated, and their perceived knowledge of the diseases were not better or worse than others. However, there were no significant differences in barriers towards getting vaccinated against influenza nor dengue.
The discussion so far indicates that there are significant differences between influenza and dengue for the five factors generally associated with vaccine uptake, thus answering the first research objective of this study. To further understand the effect of the factors that are associated with the intention to vaccinate for both the diseases, multiple regression analysis was attempted for each disease.
In the case of influenza, higher perceived susceptibility and perceived benefits of the influenza vaccine were positively associated with a higher intention of getting vaccinated. Benefits of getting vaccinated and susceptibility to influenza seem to be strong predictors of individuals’ intention to vaccinate. This is in line with other studies that have reported vaccinated participants feeling more susceptible to the disease, and perceiving more benefits from the vaccine and less barriers to take the vaccine [35
Perceived severity, knowledge of the disease, and barriers to vaccination did not have any significant relationship with the likelihood of getting the influenza vaccine. This contradicts previous studies that have found severity and knowledge to be predictors of vaccine uptake [19
]. However, studies also suggest that the predictive power of these drivers were relatively low [26
]. Some studies have also found that barriers may not be a direct predictor of vaccine uptake but may be a moderator [38
]. Future studies should investigate these relationships further.
In the case of dengue, the results mirror those found in influenza, with perceived susceptibility of the disease and benefits of the vaccine being positively associated with the likelihood of getting vaccinated against dengue. In addition, the higher the perceived barriers to vaccination, the less likely respondents are to get vaccinated. This is in line with other studies [39
]. Although a vaccine for dengue (Dengvaxia) has been available since 2017 and has been approved for use within Singapore, the vaccine is only significantly effective against two of the four strains of dengue that are not primarily found in Singapore; and the risk of contracting a more severe form of the disease is reportedly higher if there is no prior exposure to the disease before vaccination [9
]. Therefore, promotion of the vaccine has been minimal and public awareness is thought to be low, which could be a reason why barriers were significantly negatively related to uptake. Increasing public education on these preventative measures are needed in the future.
Severity and knowledge of dengue did not have any significant relationship with the likelihood of getting dengue vaccine, which is similar to the results found while investigating influenza. This could be the result of the contrast between severity being perceived as high for dengue, but susceptibility being perceived as fairly low. As preventative measures such as the “Mozzie wipeout” [40
] are promoted heavily within Singapore, people may think they are not at risk of getting dengue and thus opt to adopt preventative measures against mosquito breeding rather than vaccination methods. This answers this study’s second research objective. To summarise, the discussion shows that despite the differences in perceptions between the diseases, factors such as susceptibility, severity, and knowledge of the diseases and benefits and barriers of the vaccines do play a vital role in predicting the public’s intention towards vaccine uptake for both diseases.
Finally, there are few limitations to the study. While cues to action is a concept included in the HBM model, it was not considered because the main objective of this study was to assess personal factors that drive behavioural intention. Cues to action have been described as strategies to activate “readiness” where how-to information is provided to promote awareness and action [41
]. As such, it points more to external factors available to the individual, which is no longer within our study’s scope. In comparison to the general population, due to the door to door random sampling method, the study respondents were younger and slightly skewed towards females. However, neither age, gender, nor ethnicity influenced the predictors of vaccine uptake. Participants with higher education level were significantly more likely to take the influenza vaccine but no significant relationship was found between educational level and intention to take the dengue vaccine. Response rates were low, which could affect respondent views. However, a broad range of respondents were sampled from all over the country to lessen this bias. Intention to vaccinate does not always lead to actual behaviour; however, it is the best estimate of future vaccination. There could be many other factors influencing individuals’ vaccine uptake intentions. Future research should utilize other theoretical frameworks such as Theory of Planned Behaviour and Protection Motivation Theory to examine if the predictive power of the model is stronger.