Three-quarters of SARS-CoV-2-infected migrant workers are subclinical cases: SG study

Majority of the SARS-CoV-2 infections among foreign workers in Singapore were subclinical cases rather than symptomatic cases presented for medical consultation, according to a study conducted as part of Singapore national public health response to COVID-19.

The study was conducted between end March to July last year, when there was a surge of COVID-19 cases among foreign workers in the dormitories and a subsequent national lockdown to enforce movement restriction. A total of 198,320 migrant workers (median age 33 years, 99.8 percent male) across 43 dormitories in Singapore underwent mass screening using PCR of nasopharyngeal swab samples (in less infected dormitories) and/or serology of blood samples (in heavily infected site). [JAMA 2021;325:584-585]

“All workers with a positive PCR test result were admitted to health care facilities for isolation and treatment,” informed the researchers led by Dr Vernon Lee from the Communicable Diseases Division, Ministry of Health, Singapore.

Among the 95.1 percent of foreign workers who had been tested at least once (including 63.6 percent using PCR and 68.4 percent with serology), 111,280 dormitory residents turned out positive for COVID-19 based on PCR or serology results. This translates to an overall infection prevalence of 56.1 percent (95 percent confidence interval [CI], 55.9–56.3).

A total of 24,197 (21.7 percent of those tested positive) were classified as clinical cases — defined as individuals who presented with symptoms during clinical visits. 

On the other hand, 87,083 (78.3 percent of all infected) were considered as subclinical cases — referring to infected individuals without presentation for medical consultation but were detected during the mass screening. This group included workers who were either never presented for consultation or those who showed symptoms but were tested negative previously.

“High-density communal residences are at elevated risk of large outbreaks of respiratory disease,” pointed out Lee and co-authors. “The infection prevalence would have been severely underestimated if only persons presenting for medical consultation were tested because most infections were subclinical and were identified through mass screening.”

Among the symptomatic cases, 20 patients were admitted to the intensive care unit (ICU), corresponding to a rate of 0.08 percent of all clinical cases (95 percent CI, 0.05-0.13), including one death from COVID-19 (case-fatality rate, 0.004 percent, 95 percent CI, 0.0002–0.027 percent). The low case-fatality rate, according to the authors, compares favourably versus rates documented in other national registries in Europe and elsewhere.

“The COVID-19 outbreak among Singapore’s migrant workers was characterized by a high prevalence of infection, low morbidity with few ICU admissions, and low mortality,” noted Lee and co-authors.

“Although migrant workers are younger and generally healthy, the nationally coordinated public health response and clinical care for all cases likely contributed to favourable health outcomes,” they added.

Some subclinical cases presented with symptoms so mild that they might have miscategorized — which constitutes a limitation of the study and leading to underestimation of the prevalence.