For the first aim of this study, a desk review of adult vaccination was performed by reviewing the guidelines for three countries, USA, Saudi Arabia (KSA), and UAE. For USA, the Centers for Disease Control and Prevention (CDC) guidelines were used (16). For KSA, the schedule published by the Ministry of Health was used (17). In the UAE, there are three main health authorities: the Dubai Health Authority (DHA), responsible for the Emirate of Dubai (3.4 million; 34.7% of the population), the Abu Dhabi Department of Health (DOH/HAAD/SEHA), responsible for the Emirate of Abu Dhabi (2.9 million; 29.6% of the population), and finally, the Ministry of Health and Prevention (MOHAP) focusing on the remaining Emirates (3.5 million; 35.7% of the population) (18–20). Out of the three health authorities, only the DHA had a full adult vaccination schedule (21). The DOH's and MOHAP's schedules did not touch upon routine adult immunization; they focused on the vaccines for high-risk groups (22). Given the aims of the study, the DHA's schedule was adopted as the prototype for the UAE.

Both the global and local guidelines regarding adult vaccination were reviewed, compiled, compared, and contrasted to highlight areas of deficit. Additionally, the guidelines were simplified and presented as a short schedule to help physicians quickly determine a patient's need for a vaccine.

Thereafter, a cross-sectional, descriptive study was designed to collect original data from UAE physicians all over the country. It was conducted between the months of June and October 2020 using convenience and snowball sampling. Participants were approached through email, phone, WhatsApp, and other social media networks making use of contact details listed on the health authority websites. Any physician with at least 1 year of experience who is currently practicing in the UAE was eligible to participate. The minimum sample size needed was 385, assuming an expected prevalence of 50%, a margin of error of 5%, and 95% confidence. The number was increased by 20% to 460 to account for non-response. In total, 534 questionnaires were collected out of which 34 were excluded due to them not fulfilling the inclusion criteria.

A 40-item questionnaire was developed after reviewing the adult vaccination literature and guidelines. Due to the COVID-19 pandemic, data was collected online. Google Forms was used, ensuring the user's privacy by not collecting any personal identifiers. The self-administered questionnaire consisted of two main sections: demographics and adult vaccination knowledge. It included 5-item Likert scales, true and false questions, as well as multiple-choice questions. The questions explored physicians' (a) perceived knowledge, (b) the vaccines they would recommend at different age groups, and (c) their knowledge sources and experience with local guidelines.

The questionnaire was pilot tested several times on different physicians; all provided feedback was evaluated and incorporated, if appropriate. This study was reviewed and approved by the Research Ethics Committee at the University of Sharjah (Reference Number: REC-20-04-09-01-S). It was conducted in accordance with all relevant guidelines and regulations. Informed consent was obtained from all participants.

The data was exported from Google Forms and imported into Python 3.9 for analysis. Data cleaning and pre-processing was performed. A knowledge score was calculated by categorizing the adult vaccines into four main groups:

For every vaccine that a participant recommended, 2 points were awarded if the vaccine is recommended for all healthy adults, or 1 point awarded if it is recommended for all healthy adults lacking immunity, or 1 point deducted if the vaccine is only indicated for specific risk factors or not routinely indicated. All the points were added to calculate the knowledge score. The maximum possible score was 31 while the minimum was −11.

For univariate analysis, the normality of the knowledge score was evaluated using both Q-Q plots and a Shapiro-Wilk test. All reported percentages were calculated by excluding the missing values. All demographic variables and perceived knowledge were evaluated as predictors for the knowledge score. Bivariate analyses were conducted to identify significant predictors using Mann–Whitney U and Kruskal–Wallis H-tests, the former for binary variables and the latter for those with more than two categories. The cut-off for significance was a P < 0.05.

All determinants were categorical and hence dummy coded except for perceived knowledge. Ordinary Least Squares regression was used. Heteroskedasticity was tested for using a Studentized Breusch-Pagan test. No outliers were detected. For the linear regression model, the minimum number of cases was met, which was calculated using 50 + 8m, where m is the number of predictors. No interactions were explored. F score and R-squared values were calculated for the model. All P-values reported are two-sided and all confidence intervals are profile confidence intervals.

Adult vaccines can be divided into two main groups: those recommended for the general adult population and those indicated for individuals with specific risk factors such as chronic lung or heart disease, diabetes, compromised immune system, travel, or high-risk occupations (16). Table 1 shows the guidelines for the UAE, KSA, and USA. UAE guidelines use three age groups: 18–59, 60–64, and 65 or older. The KSA and CDC guidelines take a more granular approach and utilize five age groups: 18–26, 27–49, 50–59, 60–64, and 65 or older.

The three schedules' recommendations are aligned for influenza, Tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis (Tdap), Measles, Mumps, and Rubella (MMR), and varicella vaccines. For the pneumococcal vaccine, the CDC and UAE recommend Pneumococcal polysaccharide vaccine 23 (PPSV23) routinely while KSA recommends both PPSV23 and Pneumococcal conjugate vaccine (PCV13). As for the hepatitis B vaccine, the KSA guidelines routinely recommend it whereas the UAE and the CDC do not. Finally, for hepatitis A and Haemophilus influenzae type B (Hib), they were not discussed in the KSA guidelines and the UAE guidelines only indicated them for those with other risk factors.

Considering the unique healthcare system, demographic distribution and public health policies of the UAE, an outline of the recommended adult vaccines is presented in Table 2. It is adopted from the CDC's schedule, considering the regional and local meningococcal and hepatitis B guidelines. With regards to influenza, only the inactivated/recombinant vaccine was included for two reasons (a) the UAE's guidelines state that the live vaccine is not available (21) and (b) the CDC recommends it over the live one (16). Similarly, for the zoster vaccine, the recombinant was included given the CDC's recommendation.

Additional points and comments can be added to the schedule to make it more exhaustive; however, the goal was to present a simple yet accurate representation of the possible vaccinations at different age groups, serving as a quick reference sheet. While indications and risk factors are not highlighted, situations where a physician needs to probe further are indicated. However, still, a few comments are warranted:

Adult vaccination is yet to achieve the same level of success that childhood vaccination has. Many organizations have looked at ways to improve adult vaccine uptake rates through a variety of different frameworks, actions, policies, recommendations, and programs. Yet, its rates remain dismally low locally, regionally, and even globally. In the USA, the rates of adult vaccine coverage in the community are low across the board with the highest uptake rate being for influenza at 44.8% (23). Data from the European Union presents an equally gloomy picture, with eight countries having elderly influenza vaccination rates of <50% and reaching as low as 4.3% (24). Moreover, out of 31 surveyed advanced economies, only 12 had a comprehensive adult vaccination schedule, with most recommendations geared toward older vaccines such as influenza and hepatitis B (25). As for the MENA region, data and research regarding adult vaccination is severely lacking. Influenza vaccination rates are dismally low in most countries, including the UAE and even among high-risk groups (26). Reasons for the low rates were plentiful, from negative media coverage and fear of side effects to low recommendations by physicians (7).

VPD monitoring and seroprevalence studies in the UAE are very scarce and most do not focus on adults. However, Abu Dhabi releases a quarterly communicable disease bulletin detailing the number of notified cases in the last three months; the latest of which (covering the second quarter of 2019) showed among adults aged 25 and above: 862 cases of chickenpox, 320 cases of hepatitis B, 1,756 cases of influenza, and an average of 25 cases for each of measles, mumps, and rubella (27). The yearly numbers can reach as high as ten times, given that this data represents the number of cases for a third of the population and only for a quarter of the year. Even more worryingly is the expected increased burden due to a rapidly aging population. In 2015, only 2.3% of the UAE's population was aged 60 years or older, compared to the West, where they represented more than a fifth of the population in most countries (4). However, the UAE's elderly population is expected to increase more than five-folds in 2030, reaching 11.3%, with some models predicting it as high as 14.3%. Hence, the effects of poor adult vaccine uptake rates have been masked by a young population, though not for long. As for vaccination studies, those are not extensive and deal with either specific vaccines or specific age groups; however, their results still show a lack of vaccine uptake and a high level of disease susceptibility (13–15).

The UAE's national agenda specifies a wide-ranging work program centered around six national priorities, the fourth of which is a “world-class healthcare” (28). Prevention would be an important pillar in such a healthcare system. Not only is this a fundamental component of a healthy good life, but it is also important as improved health leads to higher workforce participation and productivity. Hence, in any country, the goal is to integrate immunization into the healthcare system as a long-term sustainable service.

In this study, the adult vaccination guidelines in the UAE were examined and compared to those in KSA and USA and the knowledge levels of UAE physicians were explored. The UAE guidelines for adult vaccination were found to be dated, unclear and not easily accessible. The guidelines additionally lacked extensive details regarding dosing, indications, and contraindications compared to the CDC's. The results also showed that physicians' knowledge of adult vaccines is lacking with most physicians being aware of this. Many were unable to recognize the vaccines recommended at every age group. Even the elderly vaccination schedule was not well-recognized, with a worryingly low number of physicians recommending the pneumococcal and influenza vaccines, the most fundamental for that age group. Future studies would have to establish whether the lower recommendation rates for the elderly stem from a lack of knowledge or a fear of side-effects in that vulnerable age group.

Very few studies have looked at adult vaccine knowledge in the region. However, AlMansoori et al. explored HPV vaccine knowledge in Al Ain city, UAE. Most knowledge questions were answered incorrectly by more than 40% of the participants and the HPV vaccination schedule was incorrectly recalled by more than 80% (29). In KSA, it was found that around three-quarters of physicians had poor knowledge regarding adult vaccines. The most cited reasons for the low adult vaccination rates were time constraints and a lack of up-to-date records (30).

It is important to highlight the possible limitations of any research. This study depended on what physicians reported without any independent verification. Moreover, convenience and snowball sampling were used, yet care was taken to ensure responses were equally distributed among the three major health authorities in the country. No information was collected regarding the dosing that physicians would use. Additionally, as with all surveys, social desirability, and recall and response bias are possible. However, the survey being completely anonymous without any personal identifiers hints at the responses being authentic.