Physical distancing measures can curb up to 99% of COVID-19 transmission: SG modelling study

Implementing a combined intervention of school closure, workplace distancing, and quarantine of infected persons and their close contacts can substantially slow down the spread COVID-19, reveals a modelling study in Singapore.

“Once community transmission has been detected … [the combined intervention] could substantially reduce the number of SARS-CoV-2 infections. We therefore recommend immediate deployment of this strategy if local secondary transmission is confirmed within Singapore,” according to the researchers led by Dr Alex Richard Cook from Saw Swee Hock School of Public Health at NUS*, Singapore.

In an epidemic simulation model, the researchers compared the impacts of four intervention scenarios — quarantine, quarantine + school closure, quarantine + workplace distancing, and a combination of all three strategies mentioned (combined intervention) — vs a baseline scenario (without any intervention). The underlying assumptions were that 7.5 percent of the infections were asymptomatic and the virus had a mean incubation time of 5.3 days. [Lancet Infect Dis 2020;doi:10.1016/S1473-3099(20)30162-6]

Based on an R₀** of 1.5, the cumulative number of infections at 80 days was estimated at a median of 279,000 (interquartile range [IQR], 245,000–320,000). This number, according to the authors, corresponds to 7.4 percent of the population.

With quarantine intervention, the median cumulative infections were reduced by 94.8 percent to 15,000 on day 80, compared with the baseline scenario.  

The addition of school closure or workplace distancing to quarantine further reduced the median cumulative infections to 10,000 and 4,000, respectively. The greatest reduction was seen when all three measures of quarantine, school closure, and workplace distancing were combined — leading to a 99.3 percent reduction in the median cumulative infections to 1,800 on day 80.

“Our findings suggest that national outbreak control is feasible provided that R₀ is low (R₀≤1.5), with a combination of the proposed intervention measures being most effective,” said Cook and co-authors.

“Quarantine and workplace distancing should be prioritized over school closure because at this early stage, symptomatic children have higher withdrawal rates from school than do symptomatic adults from work,” they added.

However, the magnitude of reduction in infection count became smaller with increasing infectivity scenarios used in the model — compared with a 99.3 percent reduction when R₀ was 1.5, the combined intervention reduced the median infection count by 93 percent when R₀ was 2.0 and by 78.2 percent when R₀ was 2.5.

“At higher infectivity scenarios, outbreak prevention becomes considerably more challenging because although effective, transmission events still occur,” the researchers explained. “These combined interventions should therefore be implemented rapidly upon confirmation of second-generation local transmission occurring within the resident population to suppress increases in the national R₀.”

The outbreak also became more challenging to control when the proportion of asymptomatic transmission was higher. Compared with a median infection count of 1,800 when assuming an asymptomatic proportion of 7.5 percent, the infection count jumped to 58,000 when the asymptomatic proportion was 22.7 percent, up to as high as 277,000 infections when the asymptomatic proportion was 50.0 percent, when maintaining the R₀ parameter at 1.5.

“Contact inhibition remains important, but the high proportion of asymptomatic and undocumented infections and missed opportunities to quarantine will make control challenging … even at a low infectivity (R₀≤1.5),” the researchers noted.

“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,” Cook explained.

“At higher asymptomatic rates, public education and case management become increasingly important, with a need to develop vaccines and existing drug therapies,” he pointed out.

Regardless of infectivity and the proportion of asymptomatic transmissions, public compliance is essential to making the containment measures work, according to the researchers.

“Current government-led outbreak control measures will only be successful with public cooperation through exercising good hygiene, infection prevention in shared spaces, and adequate education to understand when symptoms might be indicative of a potential SARS-CoV-2 infection,” highlighted Cook and co-authors.

This is particularly important for vulnerable groups such as older people (>60 years), immunocompromised individuals, and those with comorbidities and are at risk of severe complications, they stated.