Prevalence of measles antibodies among migrant workers in Singapore: a serological study to identify susceptible population subgroups

Measles is a highly contagious viral disease and poses a significant threat to public health. It is preventable by active vaccination. The measles vaccine is safe, effective and inexpensive and it has been in use since the 1960s. Under the Global Vaccine Action Plan, all World Health Organization (WHO) regions have set a measles elimination goal, but there is considerable variation in vaccination coverage by region and country [1]. The global resurgence of measles in 2018 and 2019 has highlighted the urgent need for renewed focus on measles vaccination efforts, particularly in population subgroups with suboptimal vaccination coverage [2].

The foreign workforce made up about a quarter of Singapore’s population of 5.69 million people in 2020, including 0.85 million on work permits who mainly engage in semi-skilled work [3]. The annual incidence of measles in Singapore ranged from 3 to 29 per million population during the 10-year period from 2009 to 2018 (Additional file 1: Fig. S1) [4]. Measles vaccination was made compulsory for children under the National Childhood Immunization Program in 1985 [5]. Since 2011, the measles, mumps and rubella (MMR) immunization schedule has consisted of the first dose at 12 months and the second dose at 15–18 months of age [6]. The two-dose MMR vaccination coverage ranged from 88 to 92% in Singapore residents at 2 years of age from 2013 to 2018 [7]. The high population immunity among Singapore residents is well demonstrated by serological surveys with measles prevalence of 98% in those aged 1–17 years in 2018 [8] and over 95% in adults in 2004 [9]. However, the immunity level against measles among non-residents in Singapore is unknown, particularly in vulnerable population groups who may require catch-up vaccination.

In 2019, two measles outbreaks involving six cases each were reported in two migrant worker dormitories in Singapore from 14 to 27 June and from 9 to 29 July, respectively [10]. One cluster comprised four cases from Bangladesh and two from India, and the other cluster comprised five cases from Bangladesh and one from India [10]. The median age of measles cases in each cluster was 30 years [10]. As their childhood vaccination and serology status was unknown, an added measure to minimise the risk of further transmission through ring vaccination was initiated for all close contacts of the measles cases who did not have proof of vaccination or immunity [10]. Such measles outbreak occurrences in densely populated dormitories stress the importance of profiling the prevalence of measles antibodies among migrant workers for an accurate assessment of their immunity status.

We conducted a serological study to assess the level of susceptibility to measles among migrant workers in Singapore. The findings will provide guidance for planning outbreak prevention and control programmes in the migrant worker population.

A total of 2234 migrant workers with residual sera were included in the study. Their mean age was 27.3 years (standard deviation 5.1, range 20–49). Among them, 30.4% were from India, 20.1% from Indonesia, 12.9% from Bangladesh, 12.7% from Myanmar and the remaining were from China, the Philippines and Malaysia (Table 1). All migrant workers from Bangladesh and more than 94% from China, India and Malaysia were men. All except one from Indonesia, 94.1% from the Philippines and 83.5% from Myanmar were women.

The overall prevalence of measles IgG antibodies among the migrant workers from the seven Asian countries was 90.5% (95% confidence interval [CI] 89.2–91.6%). The seroprevalence was above 80% in migrant workers from each country of origin (Fig. 1). The seroprevalence ranged from 80.3% in workers from Malaysia to 94.0% from Indonesia. The country-specific seroprevalence was above 85% among workers born in 1965–1984 and those born in 1985–1989, and it exceeded 72% for both men and women (Table 2). When aggregated across the seven countries, the prevalence of measles IgG antibodies was significantly higher in migrant workers born in 1965–1989 than those born in 1990–1999 (95.3% vs. 86.6%, p < 0.0005), whereas there was no significant difference by gender (90.8% in men vs. 89.9% in women, p = 0.508).

There were 213 samples with equivocal or negative ELISA results. Among these, 195 (91.5%) tested positive using PRNT. Of 116 IgG-equivocal and 97 IgG-negative, 115 (99.1%) and 80 (82.5%) tested positive using PRNT, respectively.

In our serological study, the overall IgG seroprevalence of measles in migrant workers from all the seven Asian countries was below the herd immunity threshold of 95% for measles [21]. Sporadic measles outbreaks can occur in susceptible individuals, particularly in those originating from countries with suboptimal vaccination coverage. The high transmissibility of measles virus and close living environments of migrant worker dormitories may have led to the two clusters of measles cases reported in 2019 in Singapore. There have been reports of measles outbreaks associated with vaccine failure or low vaccine effectiveness in other countries [22‐24].

In this study, the IgG seroprevalence was significantly higher among older migrant workers born in 1989 and earlier years. The age at vaccination is an important host factor in determining the immune response to measles-containing vaccine (MCV) [25]. Most studies on the immunogenicity of standard-titre MCV did not report on gender differences in seroconversion rates [25].

Measles is endemic in all the seven Asian countries where the migrant workers hailed from [26]. The humoral immunity against measles observed in migrant workers is more likely to be due to past infection for those born before or in the first few years when childhood measles vaccination was introduced in the countries where they hailed from. In 1995, the vaccination coverage of the first dose for measles in children aged ≤ 1 year ranged from 72% in India to 86% in Malaysia (Additional file 1: Table S2). More than two decades later, the measles vaccination coverage of the first dose in 2019 increased in all these countries except in the Philippines [27‐34] (Additional file 1: Table S2).

Upon arrival in Singapore, all migrant workers have to undergo a pre-employment health examination, which includes review of medical history, physical examination and screening, and subsequent screening [35]. There is no vaccination policy against infectious diseases such as measles for migrant workers. The measles seroprevalence of migrant workers observed in our study could be due to a combination of those with vaccine-derived and naturally acquired immunity [10].

In 2018, Singapore was verified as having achieved measles elimination by the WHO [36, 37]. However, Singapore still faces the risk of measles importations from international travellers and migrant workers [36]. The measles outbreaks in 2019 have prompted an evaluation of the need for a catch-up vaccination program for all migrant workers without documentation of receipt of two doses of MMR. Given the highly contagious nature of measles virus, a multi-pronged approach remains essential in minimizing the risk of disease transmission. Atypical clinical presentations, especially among those who have immunity to measles [38‐41], pose a challenge to early detection of cases. Hence it is important to remain vigilant, conduct thorough epidemiological investigation of suspected cases, obtain respiratory samples to perform polymerase chain reaction tests and genotyping for these cases, and implement prevention and control measures despite high seroprevalence of measles in migrant workers [10].

Approximately 82.5% of the 97 samples which had been tested IgG-negative and all except one of 116 samples tested IgG-equivocal using ELISA were found to be seropositive after subsequent PRNT. If positive PRNT results were included in the classification of results, the country-specific measles seropositivity of migrant workers would have ranged from 97.9 to 100% instead of from 80.3 to 94.0% based on ELISA alone (Additional file 1: Table S3). It is not known whether the antibody levels of individuals who tested ELISA negative but PRNT positive correlate with protection against measles, and this should be determined by long-term follow-up.

As this study was based on residual sera from a survey on latent tuberculosis, and there was no published data on distribution of migrant workers by country of origin and age group in Singapore, we were unable to assess whether the study sample was representative of the migrant workers at large. The small number of migrant workers in some age groups limited the comparison by birth cohort and led to wide confidence intervals of the seroprevalence estimates.

The IgG seroprevalence in migrant workers from each of the seven Asian countries was below the herd immunity threshold of 95% for measles. This study underscores the importance of periodic seroprevalence surveys to measure the level of immunity against measles in migrant workers, so as to identify susceptible subgroups for active vaccination.