Modeling performed on a ‘simulated Singapore’ confirmed yet again that social distancing, quarantine, school closures, and workplace distancing remain our best tools against the spread of COVID-19.

The study looked at the efficiency of using these methods, both together and individually, in tackling the early stages of a COVID-19 outbreak. These measures had the most efficiency when used together, followed by quarantine plus workplace measures, quarantine plus school closure, and then quarantine only. However, all of these scenarios were more effective at reducing the spread of COVID-19 than doing nothing.

Quarantine for the win

The team adapted an individual-based influenza epidemic simulation model, which accounted for demography, individual movement, and social contact rates in workplaces, schools, and homes, to estimate the level of human-to-human transmission of SARS-CoV-2 transmission in case containment efforts fail. The model accounted for multiple factors such as how infectious an individual is over time, the proportion of the population assumed to be asymptomatic (7.5%), and the virus’ mean incubation period.

“Should local containment measures, such as preventing disease spread through contact tracing efforts and, more recently, not permitting short-term visitors, be unsuccessful, the results of this study provide policy makers in Singapore and other countries with evidence to begin the implementation of enhanced outbreak control measures that could mitigate or reduce local transmission rates if deployed effectively and in a timely manner,” says Dr Alex Cook from the National University of Singapore, co-author of the study.

The team adapted an individual-based influenza epidemic simulation model, which accounted for demography, individual movement, and social contact rates in workplaces, schools, and homes, to estimate the level of human-to-human transmission of SARS-CoV-2 transmission in case containment efforts fail. The model accounted for multiple factors such as how infectious an individual is over time the proportion of the population assumed to be asymptomatic (7.5%), and the virus’ mean incubation period.

The team used their model to determine how many cases of SARS-CoV-2 would take hold in an interval of 80 days after the detection of 100 cases of community transmission. They worked with three pathogens with different infectivities selected based on analyses of data from people with COVID-19 in Wuhan, China. The model ran five scenarios proposed for implementation in case local containment of the virus proves unsuccessful:

• No interventions.
• Isolation of infected individuals and quarantine of their family members.
• Quarantine plus immediate school closure for 2 weeks.
• Quarantine plus immediate workplace distancing, in which 50% of the workforce is encouraged to work from home for 2 weeks.
• A combination of quarantine, immediate school closure, and workplace distancing.

Under the first scenario, the number of infections at day 80 was 279,000 (7.4% of the resident population of Singapore), 727,000 (19.3% of the population), and 1,207,000 (32% of the population) for the low-, mid- and high-infectiousness viruses respectively.

A combined intervention approach reduced the mean number of infections by 99.3%, 93.0%, and 78.2% compared to the above baseline, the team explains, for viruses with low-, mid-, and high-infectiousness respectively. The exact proportion of asymptomatic cases in the population would also influence transmission rates, the team explains. Up to 277,000 infections were estimated to occur at day 80 under a combined intervention case for an asymptomatic rate of 50%.

“If the preventive effect of these interventions reduces considerably due to higher asymptomatic proportions, more pressure will be placed on the quarantining and treatment of infected individuals, which could become unfeasible when the number of infected individuals exceeds the capacity of health-care facilities,” Dr Alex R Cook added. “At higher asymptomatic rates, public education and case management become increasingly important, with a need to develop vaccines and existing drug therapies.”

The authors note that the quality of available census data, the movements of migrant populations, and seeding with imported cases (transmissions originating from outside Singapore) would further influence infection patterns, but these were not possible to include in the modeling. Furthermore, several parameters, such as the time between symptom onset and admission to hospital, and the asymptomatic rate were based on the related SARS-CoV virus, not on the one responsible for the current outbreak, due to the availability of reliable data.

The paper “Interventions to mitigate early spread of SARS-CoV-2 in Singapore: a modelling study” has been published in the journal The Lancet.