Introduction

Vaccination stands as one of the most effective public health achievements due to its ability to prevent a large number of diseases and deaths [1]. According to the World Health Organization (WHO), vaccines prevent between 2 and 3 million deaths each year. An additional 1.5 million deaths could be avoided with improved vaccination coverage worldwide [2]. Vaccination is important throughout the life-course, starting at infancy when life-saving vaccines may be given from birth, with some vaccines requiring boosters to ensure long-term or even lifelong protection, and continuing into adulthood, especially for people with comorbidities or at older ages when immunosenescence diminishes the immune response [3]. Vaccination programs have been especially successful in infants and children, while, vaccination coverage rates among adults are low, and increasing these rates remains a significant challenge [4,5]. Therefore, adult vaccination is an imperative of the Immunization Agenda 2030, a global initiative to reduce mortality and morbidity from vaccine-preventable diseases for everyone, everywhere [1,6].

In Mexico, the national immunization program (NIP) recommends and implements vaccines based on age groups and at-risk populations. The NIP includes two separate vaccination schemes for adults, one for 20- to 59-year-olds and another for ≥60-year-olds. Both schemes cover tetanus and diphtheria, seasonal influenza, and coronavirus disease 2019 (COVID-19) vaccinations. In addition, for individuals aged 20–59 years, a measles and rubella booster is included, and for individuals aged ≥60 years, pneumococcal vaccination is included [7]. The vaccines included in the NIP are provided free of charge. However, a 2022 national health and nutrition survey reported low vaccination coverage in adults. Among adults aged ≥60 years, 49.1% had received the seasonal influenza vaccine before the last winter season and 24.4% the pneumococcal vaccine in the last year. Meanwhile, 57.3% of adults aged 20–59 years had received a tetanus toxoid-containing vaccine in the previous ten years. These data indicate that a large proportion of adults are not protected against vaccine-preventable diseases [8]. While vaccination hesitancy is an important factor explaining low coverage rates in adults in the United States and Europe [9,10], it is not a major factor in Mexico. A systematic literature review of vaccination barriers in Latin America, including studies from Mexico, identified lack of awareness of diseases and their vaccines, unfavorable socioeconomic factors, and low education levels as common obstacles affecting vaccination coverage [11]. Targeted interventions to improve awareness of vaccine-preventable diseases and vaccinations among adults may increase vaccination coverage. To develop these interventions, we first need to understand the gaps in knowledge, attitudes, and practices (KAP) and the perceptions of adult vaccination among adults vulnerable to specific preventable diseases, as well as among healthcare professionals (HCPs) managing such diseases.

We conducted an observational cross-sectional study to describe the KAP around adult vaccination in adults and HCPs in Mexico. The primary objectives were to describe the knowledge, attitudes, perceptions, and practices of preventable diseases and vaccination among (1) adults aged ≥20 years; and (2) among HCPs. The secondary objective was to describe and identify differences in vaccination KAP among adults and HCPs by sociodemographic variables.

Methods

Results

Participants and demographic data

A total of 1,169 adults from the general population participated in the study, of whom 64% were women (n = 748). The mean age was 49.8 years (standard deviation [SD] 15.8), and the largest group consisted of the 41–60 year-olds (41.8%), followed by the 18–40 years-olds (30.1%). Of these, 37.9% were unemployed, and most had either completed middle school or high school. Participants reported specific comorbidities: hypertension (24.5%), diabetes (18.8%), and obesity (10.2%), while almost half reported no comorbidities. The majority of the health services, were affiliated with either the public health services (Secretaría de Salud) or the social security system (Instituto Mexicano del Seguro Social, IMSS). Among the participants, the large majority had been vaccinated as adults. Most of them resided in either Mexico State (56%) or Mexico City (27.9%) (Table 1).

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Table 1. Sociodemographic characteristics of the adults from the general population.

https://doi.org/10.1371/journal.pone.0348625.t001

A total of 228 HCPs participated in the study, with a mean age of 36.5 years (SD 12.2) and of whom 75.9% were women (n = 173). The majority were nurses (n = 137; 60.1%), or general practitioners (n = 64; 28.1%), while few were medical residents (n = 18; 7.9%), or medical specialists (n = 9; 3.9%). All worked at a public institution (data not shown), mostly in primary care. The largest group had worked for ≤5 years in practice, followed by those who had worked for ≥20 years in practice (Table 2).

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Table 2. Sociodemographic characteristics of the HCPs.

https://doi.org/10.1371/journal.pone.0348625.t002

Knowledge of vaccine-preventable diseases and vaccines in the general population

Among the general population, almost all (95.6%) recognized the importance of vaccines, and most (87.7%) knew that adults could receive routine vaccines yet only 34.2% were informed about receiving specific vaccines in the last 5 years. As for knowledge of vaccine-preventable diseases, 68.0% reported having some knowledge about whooping cough, and 39.4% about herpes zoster (HZ), while few had knowledge about RSV. This limited knowledge about RSV was consistent with the knowledge they had about the vaccine; when presented with a list of 13 vaccines, the RSV vaccine, which is not included in the NIP, was the least well-known. To assess the accuracy of these results, we used COVID-19 as a control, which nearly every participant was familiar with (Fig 1).

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Fig 1. General knowledge of infectious diseases and vaccines among the general population and of disease impact and severity among the HCPs.

COVID-19, coronavirus disease 2019; HCP, healthcare professional; HPV, human papillomavirus; RSV, respiratory syncytial virus.

https://doi.org/10.1371/journal.pone.0348625.g001

Fewer adults knew about the infectious nature of whooping cough (33.9%), HZ (26%), and RSV (12.1%) than they did about influenza (77.6%). When asked about the vaccines against these four diseases, most had heard of the influenza vaccine, which is included in the NIP for individuals aged ≥60 years. Fewer had heard of the vaccines against whooping cough (36.3%), HZ (11.1%), or RSV (5.8%), which are not included in the NIP. Of those who knew about the whooping cough, HZ, influenza, and RSV vaccines, 59.7% (253/424), 23.3% (30/129), 58.9% (631/1,072), and 0% (0/68), respectively, learned about them through a health center. Meanwhile, 12.5% (53/424), 34.9% (45/129), 20.5% (220/1,072), and 33.8% (23/68), respectively, learned about them through a vaccination campaign or the media (radio or television). The large majority knew that adults can be vaccinated against influenza and where to obtain the vaccine. In contrast, for HZ, for which the vaccine is not included in the NIP, very few knew that adults can be vaccinated and where to obtain the vaccine (Supplementary S1 Table).

Discussion

We conducted an observational cross-sectional study to describe the KAP concerning adult vaccination among adults and HCPs in Mexico. Overall, the general population demonstrated knowledge about the importance of vaccination (95.6%). Although unfamiliar with some diseases and their causes, the general population perceived that vaccination could prevent infections and their complications. The general population trusted vaccination recommendations most when made by an HCP. HCPs were generally knowledgeable about the diseases and the recommended vaccination schedules for adults. They indicated that before recommending a new vaccine, they would need to learn more about the disease it prevents.

The vast majority of the general population had a good overall knowledge of the role of vaccines in preventing infectious diseases. They were mostly in favor of receiving vaccines, and 91.7% would get vaccinated for HZ, 90.3% for RSV, and 95.6% for influenza if an HCP recommended it. Such positive attitudes of Mexican adults toward vaccination were also found in a KAP study about dengue vaccination that included 600 adults [17] and in a study about perspectives about maternal immunization among pregnant women [18]. Despite the overwhelmingly positive attitude toward vaccination, only 29.7% reported having received the pneumococcal vaccine, and 71.3% reported having received the influenza vaccine before the last winter season. These vaccination proportions are similar to those reported in a study from 2022, which highlighted that among Mexican adults >60 years old, only 24.4% received the pneumococcal and 49.1% the influenza vaccine at the start of the previous winter season [8]. They are also similar to a study in Spain where among adults ≥65 years, 29% received the pneumococcal and 80% the influenza vaccine [19]. Among our respondents, the main factors for not getting vaccinated were, for the pneumococcal vaccine, not being eligible based on age, and for the influenza vaccine, being eligible but the vaccine was not available where they lived.

Despite an overall knowledge of vaccines and the support of vaccination in the general population, we identified substantial knowledge gaps regarding vaccine-preventable diseases and their complications. For instance, while 98.4% knew about COVID-19, and 98.6% knew a vaccine exists against COVID-19, only 39.4% knew about HZ, and only 18.6% knew a vaccine exists against HZ. In other studies also a large knowledge gap was found between the diseases, for example a study in Greece reported that 98.2% had sufficient knowledge about the COVID-19 vaccine, while only 56% had sufficient knowledge about the pneumococcal vaccine [20], In contrast, in a survey among over 8,000 adults conducted in Saudi Arabia, 64% had heard of HZ, and 58.8% had heard of an HZ vaccine [21]. Those proportions were much higher than the 39.4% and 18.6% in our participants. The difference may be partly explained by a vast difference in education level, with 98.0% of the Saudi Arabian participants having secondary or tertiary education [21], while in our participants this was only 37.5%. Another explanation for the difference is that in Saudi Arabia, vaccines were available and promoted during intensive national communication campaigns. As we found large differences in awareness of HZ and the existence of an HZ vaccine between education levels, this may explain some of the lower HZ knowledge in our participants. Another difference, although not directly comparable as ages were categorized differently, is that the participants in the Saudi Arabian study [21] were younger than those in our study (72.8% < 50-year-olds in Saudi Arabia, vs. 30.1% ≤ 40-year-olds and 41.8% 41–60-year-olds in our study). This age difference may be relevant because we found that younger participants had significantly more knowledge of HZ than older participants. Similarly, younger participants also had more knowledge of influenza, but this was not the case for RSV.

In our general population, awareness that whooping cough, HZ, and RSV diseases have infectious causes was low (12.1–33.9%), and awareness that vaccines are available against these infections, specifically targeting adults, was also low (5.8–36.3%). These findings align with a systematic review that found limited knowledge about RSV vaccination among adults [22]. For instance, a study in the United States among 827 adults at risk for RSV reported that only 43.3% of respondents had heard of RSV [23], a proportion that is almost four times higher than in our study (12.1%). The difference could be related to higher health literacy, or the promotion and availability of the RSV vaccine for adults. In general, women are more likely to accept vaccines than men. In Mexico, women traditionally visit medical units more frequently than men; and women are also responsible for the health of children and the family in general. The general population was most likely to get vaccinated if an HCP recommends a vaccine. This indicates a general trust in the advice of HCPs, which was also noted in other studies as a review of studies worldwide, including Mexico, on attitudes toward influenza vaccine uptake among adults found that trust in healthcare workers was the most important promoter of vaccine uptake [24]. Similarly, in pregnant women in five Latin American countries, including Mexico, trust in HCP recommendations was an important driver of vaccination [18]. Thus, both globally and in Mexico, the KAP of HCPs regarding adult vaccination plays a crucial role in achieving good vaccine coverage. A literature review found that worldwide, HCPs are the most trusted source for vaccination advice [25]. Another important factor in getting vaccinated was seeing others in their age group getting vaccinated. Although respondents were unaware of the vaccines’ efficacy, they felt confident if others had already received the vaccines. The primary motivation for vaccination is to avoid infections and, secondarily, to avoid complications. Most participants report attending primary care units for vaccinations, yet 12.3% are unaware of vaccines specifically targeted at adults.

As trust in HCP recommendations has been reported as an important driver of vaccination, it may be worth noting that participants in many studies report a lack of counselling on adult vaccinations by HCPs. For instance, in a KAP study in India, only 62% of adults eligible for adult vaccination were recommended adult vaccinations [26], while in Turkey, only 59% of adults were recommended adult vaccinations [27]. Also in our study, not all HCPs recommended all relevant adult vaccinations as they lacked the necessary knowledge. Hence, improvement of HCPs’ knowledge may improve recommendations and consequently vaccination rates.

The behavior of the respondents in this study, who, although they were mostly in favor of vaccination, mainly received vaccination for diseases they considered themselves susceptible to or they considered severe, e.g., influenza, is in line with the HBM. The HBM states that people are likely to engage in disease-prevention behaviors, such as vaccination, if they perceive they are susceptible to the disease, the disease is severe, and barriers are minimal [15].

We found that for influenza, age was associated with willingness to get vaccinated if a vaccine was offered. Individuals aged 18–40 or 41–60 years were more likely to get vaccinated than those over 60. For HZ and RSV, the willingness to get vaccinated was not associated with age. In a study of 1,512 Mexicans about willingness to get vaccinated against COVID-19 before the vaccine rollout, younger groups (18–24 and 25–34 years) were more likely to be willing to get vaccinated [28]. However, the data cannot be compared directly as the age groups differ.

Among HCPs, overall knowledge about adult vaccinations was good, although correct recollection of the vaccination schedules for some vaccines, namely whooping cough and HZ, was limited. Similarly, a report from the United States, where the RSV vaccine is available for adults, showed that HCPs were not fully aware of RSV vaccine recommendations [29]. In our study, important factors for HCPs to accept or recommend a vaccine included knowledge of the disease, its complications, and the vaccine’s efficacy and effectiveness. This is in line with a systematic review of studies on primary care physician vaccination behavior, which indicated that a lack of knowledge among these providers hindered making recommendations to patients [30]. Similarly, a KAP study among HCPs in Honduras about influenza vaccination found a significant association between vaccine knowledge score and attitude score [31]. Other studies have shown that HCPs expressed a desire for more knowledge of vaccine-preventable diseases and vaccines. For instance, a systematic review about KAP of RSV found that HCPs often wanted more knowledge of the disease [22]. A report about KAP of RSV among HCPs in Jordan indicated that most had low awareness of RSV or RSV vaccines and needed more information on RSV vaccines [32]. At the time of this study, in 2024, RSV vaccines were not yet available in Mexico for adults. Therefore, we did not assess the HCPs’ specific knowledge of RSV and recommendations. However, educating HCPs about RSV and RSV vaccination before rollout may improve vaccine recommendations and uptake. Education is especially important, as misconceptions about vaccination-associated adverse events can greatly reduce vaccination rates. This was reported for influenza vaccination coverage among 947 HCPs, where vaccination coverage of HCPs was halved in 2018 compared to 2017 due to misconceptions [31]. Although almost half of the HCPs had over five years of work experience, very few reported ever treating patients with whooping cough, RSV, or HZ. There were more nurses than medical doctors among the HCPs, which explains why most had never treated patients with these diseases. Nonetheless, the HCPs favored vaccinations against these diseases and recommended that the population follow age-specific recommendations.

This study has several limitations. Firstly, it used surveys, which carry the risk of classification bias if questions are suggestive and of interviewer bias. To minimize this risk, we only hired interviewers trained in conducting surveys. Secondly, the higher number of nurses compared to medical doctors among HCPs could have biased the outcomes. This is evident in the high proportions that had never treated a whooping cough, HZ, or RSV case, as treatment is provided by medical doctors. Therefore, caution is advised when interpreting the results about disease burden and treatment. However, the findings about the vaccines and vaccination program are particularly relevant as in Mexico, nurses generally manage the vaccination program. Thirdly, when approaching people to participate in surveys, there is a risk of selection bias. To reduce this risk, we calculated in advance adequate sample sizes for both participant groups. Fourthly, since sample size was based on the feasibility of recruitment, the age distribution (i.e., the proportion of the population aged 50–59 years or ≥60 years) in the study sample may not reflect the actual age distribution in Mexico. Fifthly, as a self-reported study, recall bias cannot be excluded, nor are the responses verified, so social desirability bias cannot be excluded either. As the vaccination cards were not available, we were unable to assess the actual vaccine coverage or validate utilization responses by comparing them with the data on the cards. Hence, vaccination rates should be interpreted with caution, as studies have shown that these biases can affect the outcome in both direction and magnitude [33,34]. Sixthly, although we had sufficient power from the large sample size, and one of the strengths of the study was sample representativeness, we could not perform multivariable logistic regression to adjust for confounding due to the structure of the data. Lastly, as the study involved Mexico City and only two of the 31 Mexican states, the results cannot be generalized to Mexico as a whole.

Conclusion

Our results underscore the need to strengthen health education and communication strategies aimed at the adult population, and to address barriers that hinder vaccination access, particularly for less well-known vaccines. Improving knowledge among adults may increase vaccine acceptance and coverage. Additionally, providing HCPs with more knowledge of vaccines and vaccination recommendations could strengthen their awareness to recommend these vaccines. Our findings are important for supporting adult disease awareness initiatives and decision-making in Mexico and other Latin American countries. Efforts to provide tailored information on vaccine-preventable adult diseases may help HCPs promote immunization, thereby reducing vaccine-preventable diseases and enhancing public health.

Supporting information

Acknowledgments

The authors would like to thank Enovalife Medical Communication Service Center for editorial assistance, publication coordination and writing support (Esther Van de Vosse [EV Science Consultant]), on behalf of GSK.