A combined approach of physical distancing interventions, comprising quarantine (for infected individuals and their families), school closure, and workplace distancing, is most effective at reducing the number of COVID-19 cases, according to a modeling study in a simulated Singaporean population. While less effective than the combined approach, quarantine plus workplace measures presented the next best option for reducing SARS-CoV-2 cases, followed by quarantine plus school closure, and then quarantine only. All intervention scenarios were more effective at reducing cases than no intervention.
Transmission electron micrograph of SARS-CoV-2 virus particles, isolated from a patient. Image credit: NIAID.
The study, led by National University of Singapore’s Dr. Alex Cook, is the first of its kind to investigate using these options for early intervention in Singapore using simulation.
Despite heightened surveillance and isolation of individuals suspected to have COVID-19 and confirmed cases, the risk is ongoing, with the number of cases continuing to increase in Singapore. Schools have not been closed, and workplace distancing is recommended, but it is not national policy.
Dr. Cook and colleagues found that the combined approach could prevent a national outbreak at relatively low levels of infectivity (basic reproductivity value R0 = 1.5), but at higher infectivity scenarios (R0 = 2.0, considered moderate and likely, and R0 = 2.5, considered high) outbreak prevention becomes considerably more challenging because although effective at reducing infections, transmission events still occur.
“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,” Dr. Cook said.
To assess the potential impact of interventions on outbreak size, should local containment fail, the researchers developed 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 likelihood of human-to-human transmission of SARS-CoV-2.
Model parameters included how infectious an individual is over time, the proportion of the population assumed to be asymptomatic (7.5%), the cumulative distribution function for the mean incubation period (with the virus that causes SARS and the virus that causes COVID-19 having the same mean incubation period of 5.3 days), and the duration of hospital stay after symptom onset (3.5 days).
Using this model, the scientists estimated the cumulative number of SARS-CoV-2 infections at 80 days, after detection of 100 cases of community transmission.
In addition to a baseline scenario, which included no interventions, four intervention scenarios were proposed for implementation after failure of local containment:
(i) isolation of infected individuals and quarantine of their family members (quarantine);
(ii) quarantine plus immediate school closure for 2 weeks;
(iii) quarantine plus immediate workplace distancing, in which 50% of the workforce is encouraged to work from home for 2 weeks;
(iv) a combination of quarantine, immediate school closure, and workplace distancing.
These interventions follow some policy options currently being undertaken (quarantine and some workforce distancing) by the Singaporean Ministry of Health, as standard interventions for respiratory virus control.
For the baseline scenario, when R0 was 1.5, the median cumulative number of infections at day 80 was 279,000, corresponding to 7.4% of the resident population of Singapore.
The median number of infections increased with higher infectivity: 727,000 cases when R0 was 2.0, corresponding to 19.3% of the Singaporean population, and 1,207,000 cases when R0 was 2.5, corresponding to 32% of the Singaporean population.
Compared with the baseline scenario, the combined intervention was the most effective, reducing the estimated median number of infections by 99.3% when R0 was 1.5 (resulting in an estimated 1,800 cases).
However, at higher infectivity scenarios, outbreak prevention becomes considerably more challenging.
For the combined approach scenario, a median of 50,000 cases were estimated at R0 of 2.0 (a reduction of 93.0% compared to baseline) and 258,000 cases at R0 of 2.5 (a reduction of 78.2% compared to baseline).
The team also explored the potential impact if the proportion of asymptomatic cases in the population was greater than 7.5% (the proportion of people who are able to transmit despite having no or mild symptoms).
Even at a low infectivity (when the R0 was 1.5 or lower), a high asymptomatic proportion presents challenges.
Assuming increasing asymptomatic proportions up to 50•0%, up to 277,000 infections were estimated to occur at day 80 with the combined intervention, relative to 1,800 for the baseline at R0 = 1.5.
“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. Cook said.
“At higher asymptomatic rates, public education and case management become increasingly important, with a need to develop vaccines and existing drug therapies.”
The study was published in The Lancet Infectious Diseases.
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Joel R. Koo et al. Interventions to mitigate early spread of SARS-CoV-2 in Singapore: a modelling study. The Lancet Infectious Diseases, published online March 23, 2020; doi: 10.1016/S1473-3099(20)30162-6
