Abstract
BackgroundSince the last local case of diphtheria in 1992, there had not been any case in Singapore until an autochthonous case was reported in 2017. This fatal diphtheria case of a migrant worker raised concerns about the potential re-emergence of locally transmitted toxigenic diphtheria in Singapore. We conducted a seroprevalence study to assess the immunity levels to diphtheria among migrant workers in Singapore.MethodsResidual sera from migrant workers who hailed from Bangladesh, China, India, Indonesia, Malaysia, Myanmar and the Philippines were tested for anti-diphtheria toxoid immunoglobulin G (IgG) antibodies. These migrant workers previously participated in a survey between 2016 and 2019 and had provided blood samples as part of the survey procedure.ResultsA total of 2176 migrant workers were included in the study. Their overall mean age was 27.1 years (standard deviation 5.0), range was 20–43 years. The proportion having at least basic protection against diphtheria (antitoxin titres ≥ 0.01 IU/ml) ranged from 77.9% (95% confidence interval [CI] 72.8 – 82.3%) among migrant workers from Bangladesh to 96.7% (95% CI 92.5 – 98.6%) in those hailing from Malaysia. The proportion showing full protection (antitoxin titres ≥ 0.10 IU/ml) ranged from 10.1% (95% CI 6.5 – 15.4%) in Chinese workers to 23.0% (95% CI 17.1 – 30.3%) in Malaysian workers. There were no significant differences in the proportion with at least basic protection across birth cohorts, except for those from Bangladesh where the seroprevalence was significantly lower in younger migrant workers born after 1989.ConclusionsThe proportions having at least basic protection against diphtheria in migrant workers from five out of seven Asian countries (India, Indonesia, Malaysia, Myanmar and the Philippines) were higher than 85%, the threshold for diphtheria herd immunity. Seroprevalence surveys should be conducted periodically to assess the level of immunity against diphtheria and other vaccine preventable diseases in migrant worker population, so that appropriate interventions such as booster vaccination can be implemented proactively to prevent sporadic outbreaks.
Highlights
Diphtheria is a severe bacterial infection caused by toxin-producing strains of Corynebacterium diphtheriae
Residual sera were available from 2191 migrant workers who participated in the survey on latent tuberculosis and provided full consent and partial consent for their data and residual sera to be used for future research
A total of 2176 residual sera of migrant workers from seven Asian countries were included in the study; 672 (30.8%) from India, 434 (20.0%) from Indonesia, 289 (13.3%) from Bangladesh, 284 (13.0%) from Myanmar, 178 (8.2%) from China, 167 (7.7%) from the Philippines and 152 (7.0%) from Malaysia (Table 1)
Summary
Diphtheria is a severe bacterial infection caused by toxin-producing strains of Corynebacterium diphtheriae. There has been a drastic reduction in morbidity and mortality after the introduction of diphtheria toxoid-containing vaccine, diphtheria remains a public health issue in areas with low vaccination coverage [2]. According to a World Health Organization (WHO) manual for management and control of diphtheria, a minimum immunity rate of 90% in children and 75% in adults is required for diphtheria elimination [4]. Inadequate homogeneous coverage with three doses of diphtheria toxoid-containing vaccine across countries and populations, and waning vaccine immunity in adults have resulted in recent resurgences of diphtheria [2]. Since the last local case of diphtheria in 1992, there had not been any case in Singapore until an autochthonous case was reported in 2017 This fatal diphtheria case of a migrant worker raised concerns about the potential re-emergence of locally transmitted toxigenic diphtheria in Singapore.
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