Study: Social distancing might reduce COVID-19 spread by as much as 99%

March 24 (UPI) -- Social distancing might be the most effective way to reduce spread of the new coronavirus, a study published Tuesday in The Lancet suggests.

Using an outbreak model, researchers in Singapore found that a combined approach of physical distancing -- quarantining infected individuals and their families, closing schools and practicing workplace distancing -- reduced the estimated median number of COVID-19 cases in the population by as much as 99.3 percent.

The researchers cautioned that, because little is known about the transmissibility of the virus -- they used data from the 2004-05 SARS outbreak to create their model -- the real effects of social distancing are unclear.

"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. Alex R. Cook, vice dean for biostatistics and modelling in the Saw Swee Hock School of Public Health at the National University of Singapore, said in a press release.

Just one tool

On Monday, the World Health Organization warned that social distancing measures are not enough to stop the spread of the new coronavirus, which has infected nearly 400,000 people worldwide, including more than 43,000 in the United States.

Several cities and states across the country -- including California, Illinois and New York, as well as parts of Florida and Texas -- have urged "non-essential workers" to stay home and residents to practice social distancing to stem the spread of the virus.

On Sunday, Italian officials banned all travel within the country and shut nonessential industries, after more than 600 people died from COVID-19 over a 24-hour period. Italy has been the second-hardest hit country so far during the pandemic, with more than 60,000 confirmed cases and 6,000 deaths.

France and Spain have introduced similar measures as their cases and deaths have continued to rise, while Britain instituted a nationwide lockdown Monday after public health experts in the country had pushed the government to do so for days.

"Asking people to stay at home and other physical distancing measures are an important way to slow down the spread of the virus and buy time, but they are defensive measures that won't help us to win," WHO director-general Tedros Adhanom Ghebreyesus said at a press briefing Monday.

He added that the world's hardest-hit countries also need to quarantine -- and treat -- all confirmed cases while performing contact tracing and subsequent quarantining of all close contacts.

Transmission rates matter

For the new study, researchers developed an individual-based flu epidemic model to estimate the likelihood of human-to-human transmission of COVID-19, and then assess the potential impact of social distancing on the outbreak.

Model parameters included how infectious an individual is over time, how much of the population could be asymptomatic, the incubation period for the virus and the duration of hospital stay after symptom onset.

With the model, researchers estimated the cumulative number of COVID-19 infections at 80 days, after detection of 100 cases of community transmission.

Given the limited knowledge about how contagious the virus is, the researchers calculated the impact of social distancing based on three levels of virus transmissibility: low infectiousness, with a virus reproduction number of 1.5; moderate and likely infectiousness, with a reproduction number of 2.0; and high infectiousness, with a reproduction number of 2.5.

The basic reproduction numbers were selected based on analyses of data from people with COVID-19 in Wuhan, China, where the outbreak started.

Finally, they modeled case totals based on the following scenarios: no distancing intervention; isolation of infected individuals and quarantine of their family members, or quarantine; quarantine plus immediate school closure for two weeks; quarantine plus immediate workplace distancing, in which half of the workforce is encouraged to work from home for two weeks; and, a combination of quarantine, immediate school closure and workplace distancing.

They also assumed that 7.5 percent of the infected population was asymptomatic, but still contagious. At this point, it's unclear how many people have been sickened with COVID-19 but display no symptoms.

With no intervention and relatively low levels of virus transmissibility, the researchers found that, at day 80, 7.4 percent of the resident population of Singapore's would be infected. At moderate levels of transmissibility, 19.3 percent of Singapore's population would be infected, and at high transmissibility, 32 percent of Singapore's population would be infected.

The combined distancing intervention, researchers said, was most effective at reducing the number of infections at all three levels of transmissibility: 99.3 percent at low, 93 percent at moderate and 78 percent at high. And all intervention scenarios were more effective at reducing cases than no intervention.

Cook also warned that if distancing interventions are reduced because more people are asymptomatic -- and appear not have COVID-19 -- quarantine and treatment of infected people "could become unfeasible when the number of infected individuals exceeds the capacity of healthcare facilities."