During August 2017–January 2018, more than 700,000 forcibly displaced Rohingyas fled from violence in Rakhine State, Myanmar, to Cox’s Bazar, Bangladesh, joining more than 200,000 Rohingyas already living in refugee camps in the region [1]. Complex public health issues of concern—including poor sanitation, malnutrition, overcrowding, and lack of access to safe water and healthcare—increase susceptibility to infectious diseases, particularly among children [2]. World Health Organization (WHO) guidelines emphasize the importance of using multiple modes of vaccination delivery in complex emergencies, including campaigns, vaccination upon entry, and routine immunization services. Prior to August 2017, routine services were available at a subset of health facilities in registered camps; however, these facilities had limited reach and lacked resources and infrastructure to serve new arrivals [3].

In September and November 2017, the first suspected measles and diphtheria cases were reported in Cox’s Bazar [4]. In addition to the delivery of routine services, immunization campaigns are crucial for controlling outbreaks of vaccine-preventable diseases (VPDs). Two doses of measles-rubella (MR) vaccine and three doses of diphtheria-tetanus containing vaccine (DTCV) are recommended for providing immune protection against these VPDs [5, 6]. Therefore, 2 rounds of MR and 3 rounds of DTCV campaigns as well as 2 campaign rounds of oral cholera vaccine (OCV) (as a preventive measure) were conducted in Cox’s Bazar during September 2017–March 2018, primarily targeting children younger than 15 years (S1 Table). Despite these efforts, outbreaks of measles and diphtheria continued in registered camps and makeshift settlements (MSs). As of August 28, 2018, more than 3,000 suspected measles cases and 8,000 suspected diphtheria cases had been reported among Rohingyas in Cox’s Bazar [4, 7].

Little is known about vaccination coverage among Rohingya children or their access to routine immunizations or vaccination campaigns prior to their arrival in Cox’s Bazar. During the September–December 2017 vaccination campaigns in the refugee camps and MSs, vaccination teams observed some hesitancy among Rohingyas to participate in vaccination campaigns. As a result, focus group discussions and key informant interviews with caregivers of children living in the MSs, camp leaders, religious leaders, and teachers were conducted to identify potential barriers to participation in the vaccination campaigns. Findings from the assessment highlighted several barriers, including beliefs about vaccination causing people to become Christian, concerns about multiple vaccinations given on the same day, worries about vaccination side effects, and lack of sensitivity to cultural gender norms at vaccination sites [8].

As a follow-up to the qualitative assessment, we conducted a cross-sectional serologic and vaccination coverage survey in 2 areas of Cox’s Bazar: Nayapara Registered Refugee Camp (henceforth referred to as Nayapara) and the MSs in April–May 2018. Nayapara, established in 1992, is a registered refugee camp managed by the United Nations High Commissioner for Refugees (UNHCR) with an estimated population just under 23,000 [9]. MSs are informal, self-settled areas with a population of more than 850,000, the majority of whom arrived after August 2017 [9]. Because of its longstanding status, Nayapara has better infrastructure than MSs, including provision of routine immunization services, whereas MSs were—until recently—uninhabited land and therefore have limited facilities providing routine immunizations [10]. We aimed to estimate population immunity to VPDs after vaccination activities in Nayapara and MSs to identify any remaining immunity gaps specifically among Rohingya children 6 months to 14 years.

This study is reported as per the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guideline (S1 STROBE Checklist). A prospective protocol and analysis plan were used in designing the study (S1 Protocol).

These findings suggest that although multiple rounds of MR and DTCV vaccination campaigns in MSs and Nayapara likely improved immunity to VPDs among children 1 to 14 years old, immunity gaps remain in both camps, particularly for diphtheria in MSs among younger children. To a lesser extent, immunity gaps also remain for tetanus in both age groups and for rubella in the younger age group in MSs. The presence of these immunity gaps is supported by continued reporting of suspected measles and diphtheria cases. Based on results from the survey, very few were vaccinated at the border, and vaccine uptake during campaigns may have been hindered by caregivers’ lack of awareness of campaigns and unavailability of children during campaigns because of school, daycare, or labor. Prior to this assessment, there was limited information about vaccination coverage among Rohingya children and their access to routine immunization services and vaccination campaigns before their arrival to Cox’s Bazar. This serosurvey and vaccination coverage survey provides a timely evaluation of remaining immunity gaps for VPDs among this population.

Significant differences in seroprotection were observed comparing older and younger children among the recently arrived; seroprotection for both rubella and diphtheria was higher among 7- to 14-year-olds compared to 1- to 6-year-olds in MSs. Additionally, a larger proportion of 7- to 14-year-olds born in Myanmar reported receipt of injections in Myanmar and proof of BCG vaccination compared to 1- to 6-year-olds. This difference may be attributable, in part, to the declining access to health services in Myanmar in recent years due to violence against the Rohingya population in Rakhine State [13]. Routine immunizations in Myanmar are usually administered within the first 2 years of life, including BCG, which is given within 6 weeks of birth and can be used as a marker for access to and utilization of routine services [14]. Thus, children 7 to 14 years old in MSs, the majority of whom were born in Myanmar, may have been more likely to receive all recommended vaccines in the national schedule than children 1 to 6 years old, who were born more recently during the violence and political unrest. It is also possible that children 7 to 14 years old had higher seroprotection simply because of increased exposure to VPDs and therefore more natural immunity than children 1 to 6 years old.

In MSs, children 1 to 6 years old with evidence of BCG vaccination were more likely to be seroprotected against diphtheria, which is not surprising because children who received BCG vaccine likely had better access to and acceptance of other vaccines [15, 16].

Seroprotection increased with the number of MR and DTCV doses received during campaigns in MSs. Although campaign coverage from recall and card was consistent with seroprotection for most antigens, MR campaign coverage for at least one dose in MSs among 7- to 14-year-olds (82%) was lower than measles seroprotection (99%) and rubella seroprotection (96%). This difference might be explained by protection obtained through exposure to the ongoing transmission of measles virus (>3,000 suspected measles cases during September 2017–July 2018) and potentially rubella virus, previous outbreaks of measles in Myanmar, and MR vaccination in Myanmar prior to arriving in Bangladesh [7]. Conversely, DTCV campaign coverage for at least two doses among 1- to 6-year-olds in MSs (88%) was higher than diphtheria seroprotection (63%) and tetanus seroprotection (76%). Seroprotection may have been lower than expected because the third round of the DTCV campaign may have been conducted too soon after the second campaign (approximately 4 weeks) for children above the age of 12 months. WHO vaccination guidelines recommend administering the third dose of a diphtheria-containing vaccine at least 6 months after the second dose and a third dose of tetanus-containing vaccine at least 1 year after the second dose in order to mount durable protection [17–19]. While the DTCV survey coverage in MSs was similar for both children aged 1 to 6 years and those aged 7 to 14 years, the higher tetanus and diphtheria seroprotection in the 7- to 14-year-olds supports the hypothesis that this age group had previous access to routine immunizations prior to arrival in Bangladesh.

This study had certain limitations. Recall of campaign participation may be inaccurate. The study team attempted to minimize recall inaccuracies by using a calendar that incorporated vaccination campaigns alongside co-occurring migration (e.g., August and September mass in-migration), religious holidays (e.g., Eid), and cultural events (e.g., agricultural harvests). Findings from the study may not be generalizable to the larger Rohingya population because of the exclusion of 7- to 14-year-olds in Nayapara due to Ramadan as well as nonresponse—particularly in MSs among the 7- to 14-year-olds, who accounted for 89% of children not at home during the survey. The rubella and measles antibody MBA validation process is ongoing but nearly complete. The diphtheria antibody MBA is currently in the validation process and may not have direct correlation with protection, which is toxin neutralization measured by the Vero cell neutralization assay. Toxin neutralization can be due to antibodies other than immunoglobulin G (IgG); however, cross-reacting material (CRM)-conjugated vaccines were not used in vaccination campaigns, so interference from other vaccines is unlikely. Additionally, the tetanus MBA has already been validated, and diphtheria and tetanus results were similar [20]. Lastly, typically higher antibody levels for tetanus and diphtheria correlate with higher probability and duration of protection, but it is possible that an individual with low antibody levels is protected whereas an individual with high antibody levels could still acquire symptomatic disease [10]. The serologic results for any antigen provide a snapshot in time of immunity, and because antibody levels can wane over time, immunity gaps may be larger now than reported in this analysis.

WHO’s “Vaccination in Acute Humanitarian Emergences: a Framework for Decision-Making” provides guidance to governments and international partners on how to determine the risk of VPD outbreaks, which vaccines to deliver, and how to deliver them in the context of availability of human resources, cultural considerations, and appropriate social mobilization strategies [21]. In a rapidly evolving emergency situation, however, it can be difficult to operationalize these guidelines in a timely manner, as evidenced by the small proportion of children who received vaccination upon entry to Bangladesh and the remaining immunity gaps in the population, despite multiple vaccination campaigns. Multiple dose vaccinations are particularly difficult to deliver by campaigns alone, not only because of the intensity of resources necessary but because of campaign fatigue, a constant new cohort of children arriving who require multiple doses for full protection, and unique sociocultural issues. It is challenging to fully understand and adequately prepare for societal and cultural beliefs towards vaccination. Just prior to the survey, a rapid behavioral assessment, which included focus group discussions and key informant interviews, was conducted in 3 Rohingya camps. Findings from the assessment highlighted several major barriers to achieving high campaign coverage, including fear and misconception of becoming a Christian due to vaccination; safety concerns, including receipt of multiple vaccinations on the same day; and lack of sensitivity to cultural gender norms during vaccination procedures [8]. These findings, along with specific recommendations, were communicated to stakeholders and should continue to be considered as the Rohingya crisis evolves.

Children may still be at risk for future infections because of ongoing transmission of diphtheria and other infectious diseases in Rohingya camps and settlements. The guidelines on vaccination in acute humanitarian emergencies focus mainly on the use of mass vaccination campaigns, with instruction to resume or establish routine immunization whenever possible [21]. Although rapid scale-up and strengthening of existing routine immunization services is challenging because of political and resource constraints in Rohingya camps and settlements, reaching children under 2 years and delivering missed doses to older children to close these immunity gaps and prevent future VPD outbreaks is critical. Delivery of routine services is particularly important for closing gaps when serial doses are needed (e.g., 3 doses of DTCV). In emergency settings, vaccination upon entry, mass vaccination campaigns, and rapid scale-up of routine immunization services with strengthened social mobilization—including community leader involvement—is essential for protecting all children.

Based on the findings from this survey, despite multiple rounds of vaccination campaigns, VPD immunity gaps exist among children in both camps, particularly for diphtheria in MSs, which requires serial vaccinations to achieve maximum protection. In addition to scaling up routine immunization services, an additional tetanus-diphtheria campaign may be necessary in MSs to quickly address these remaining immunity gaps among children aged 1 to 14 years. Future campaigns and routine immunization services should be provided with respect to the sociocultural issues specific to the Rohingya population.