Full immunization coverage is one of the key public health measures to prevent morbidity and mortality worldwide ¹ . Yet, the effect of immunization does not only depend on its completeness but also on its timely administration ² . If a child is not immunized appropriately or if immunization is delayed, then the child is at a higher risk of falling sick from a preventable disease ³ . According to the World Health Organization (WHO), an invalid vaccine dose is defined as any dose of vaccine administered earlier than the recommended age ⁴ . Risk factors for a child to receive untimely vaccine doses can be multifold and may relate to existing immunization practices, logistical aspects of providing vaccines, perceived contraindications, beliefs and attitudes towards vaccines or socioeconomic determinants ^{5– 8} . Demographic characteristics, including the age, educational level and employment status of children’s caregiver, can be crucial for a child receiving timely vaccination or not ^{9, 10} .

Demographics in Lebanon are dynamic and underlie constant changes due to the social, economic and political fluctuations inside and outside the country ¹¹ . The influx of Syrian refugees is the most recent shift, impacting on various spheres of life of the Lebanese population ¹² . With regard to immunization coverage, the demand for vaccines increased greatly as nationwide vaccination campaigns were introduced to ensure optimal coverage levels and to prevent disease outbreaks ¹³ . Despite the prominent resilience of the Lebanese health care system, low immunization rates among Syrian refugee children persisted and are indicative of deficiencies in managing routine vaccination coverage during the crisis ^{14, 15} . Furthermore, the variety of providers offering vaccines, as well as the multiplicity of schedules used, may result in different practices around the timeliness and simultaneity of vaccination.

Few studies exist that show how knowledge and awareness among parents influence a child’s immunization status in Lebanon ¹⁶ and no study on vaccination timeliness exists. Therefore, it is crucial to extend our understanding about attitudes and behaviors to establish evidence-informed interventions. In the region, studies have shown that parental knowledge and practices are associated with complete vaccination ¹⁰ . In addition, the integration of immunization services in antenatal care may enhance vaccination take-up and timely administration ¹⁷ .

In 2016, a district-based immunization coverage cluster survey was conducted in Lebanon following the Syrian crisis; it included potential determinants for vaccination ¹⁸ . In order to further understand gaps in vaccine administration, this study analyzed vaccination timeliness and adherence to the national vaccination schedule set by the Ministry of Public Health (MoPH). Furthermore, key socio-demographic, knowledge, belief and practice factors associated with timely vaccination among Lebanese and Syrian children aged 12–59 months old were investigated.

Of the 500 randomly selected children, seven who were neither Lebanese nor Syrians were excluded, leading to a final sample for analysis of 493 children. Their characteristics are described in Table 2.

This study is the first to assess the age-appropriate vaccination coverage and its predictors among a subset of Lebanese and Syrian children aged 12–59 months residing in the Lebanese communities in 2016. Vaccination programs aim at attaining the highest level of protection against vaccine-preventable diseases at a young age together with high immunization coverage rates ²⁴ . Routine vaccination timeliness and completeness are still public health challenges. Evaluating age-appropriate vaccination provides valuable insights even in populations with very high up-to-date immunization coverage ⁹ . Currently, timeliness is not routinely used as an indicator to evaluate immunization programs in Lebanon. Special efforts need to be directed towards the inclusion of timeliness of vaccination as another indicator of the performance of the Expanded Programme on Immunization in Lebanon considering its crucial role in improving children’s health and survival and reducing the risk of disease in children ²⁵ .

In this study, the age-appropriate vaccination coverage has been described and graphically visualized using the inverse Kaplan–Meier survival curve. About 75% of children, among those included in this survey sub-sample, received the first doses of polio and DTP vaccines by 75 days, but few were immunized with these vaccines at the recommended age. The situation was even more worrisome in the case of the third doses of polio, HepB and DTP vaccines; while about 65% of children received these doses by 195 days; only 1 in 4 received them at the appropriate age. The doses of MCV vaccine were more timely than the third doses of polio, HepB and DTP, which can be because MCV is a two-dose series and has a longer interval between the doses. Despite the efforts made to ensure MCV timeliness, measles and mumps outbreaks emerged in Lebanon in early 2018, which may be attributed to the challenge of having a large influx of Syrian refugees ^{11, 26} . With the existence of such a pool of vulnerable children, outbreaks may occur much faster when untimely vaccination is coupled with low immunization coverage and reduced vaccine effectiveness ²⁷ . The difference in vaccination schedules and the application of different vaccines or vaccine combinations may also contribute to the variation in timeliness ²⁸ . The results of this analysis are in line with the results of previous studies in other countries that showed relatively low proportions of children vaccinated at the appropriate age ^{29, 30} .

This study identified several factors, including socio-demographic, knowledge, beliefs and practices associated with age-appropriate vaccination; however, this association differed between the types and doses of vaccine. First, receiving HepB vaccine birth dose from the hospitals was a positive predictor for age-specific vaccination, compared to those who received vaccination in a health facility. This may reflect that health services, mainly having a better utilization of vaccination services, were more accessible by mothers who gave birth at a hospital, especially for HepB birth dose to be given within 3 days of birth ³¹ . Second, erroneously believing that children’s vaccination is up-to-date for their age was negatively associated with the administration of timely vaccines. A mother’s communication with health providers might be hindered by a low education level influencing negatively the immunization through the wrong beliefs ^{28, 32, 33} . Poor knowledge of immunization schedules among mothers may also explain this negative association. Third, using the information provided by schools and nurseries to decide about the child’s vaccination rather than obtaining this information from a private physician or health facility staff was also a risk factor. Healthcare facilities and private clinics are responsible to a greater extent for the correct administration of vaccinations. These providers are expected to play an important role in determining invalid vaccinations ³⁴ . Further studies are needed to examine the factors associated with vaccination timeliness in Lebanon, as factors related to timely vaccination were not identified for all vaccine doses.

Our study was subjected to important limitations. First, the timeliness assessment was done on a small sub-sample of 493 children, because children’s vaccine status was assessed case by case. Only children with available vaccination cards with administration dates were included as this was the only way to obtain accurate information on age at vaccination. While the exclusion of caregiver's recall reduced recall bias, purely relying on vaccination cards may have resulted in an underestimation of vaccination delay, particularly those children without an available vaccination card might be more prone to untimely vaccination. Another obstacle during the assessment was the illegible cards. Although vaccination cards were assumed to have the most valid reporting of received vaccination, the handwriting of health workers created difficulty to accurately determine the date of vaccine administration. Second, the definition of timeliness was specifically developed for this assessment based on expert opinion because diverse practices exist among providers in the Lebanese vaccine system. The diversity in practice can be related to different available vaccination schedules and vaccines or vaccine combinations. These factors jeopardize the generalizability and comparability of results to other assessments, and interpretations should always consider the context of this study. Despite these limitations, our study is the first one to show that untimely vaccination is of concern in Lebanon, and to shed light on factors related to timely vaccination that could be useful when devising strategies to improve age-appropriate vaccination.