Most projections indicate that social distancing – from mobility restrictions to complete lockdowns -- may take many weeks, possibly even months, a potentially devastating outcome for social and economic stability. One of the challenges is that the sick cannot be selectively isolated, since many of the spreaders remain pre-symptomatic for a period ranging from several days to as much as two weeks - invisible spreaders that continue to be socially active. Hence, it seems that without a population-wide lockdown isolating the carriers cannot be achieved effectively.
To bypass this challenge, Bar-Ilan University researchers, led by Dr. Baruch Barzel of the Department of Mathematics, have devised a strategy based on alternating lockdowns: first splitting the population into two groups, then alternating these groups between lockdown and routine activity in weekly succession. The researchers conclude that, together with isolation of the symptomatic spreaders and the adoption of everyday prophylactic behaviors, this strategy can help defeat the virus, while sustaining socio-economic activity at a 50% level.
In the alternating lockdown routine, society will be partitioned into two groups, with little interaction between them -- one half active this week, and the other active only the next. This will already slow the spread, but its main advantage is that it helps isolate the invisible spreaders, such as those who are pre-symptomatic carriers still in the incubation period.
"Consider an individual who became infected during their active week. They are now in their pre-symptomatic period – the most dangerous stage, in which they are invisible spreaders. The crucial point is that, according to the alternating lock-down routine they are now scheduled to enter their lock-down phase" explains Dr. Barzel.
"Staying at home for another week, they will most likely begin to exhibit symptoms, and therefore remain in isolation until full recovery. Indeed, if following a week of lock-down they show no symptoms, they are most likely uninfected and can partake in social and professional activities during their active week. Therefore, alternating lockdown with full isolation of symptomatic spreaders ensures that at all times, the majority of invisible spreaders are inactive, as their incubation period is naturally directed towards their lockdown phase."
At present, most people are extremely cautious, and will refrain from any contact with someone showing symptoms. "Therefore, we believe that it is the invisible spreaders that are the main contributors to the proliferating case count. Our strategy is precisely directed at this challenge -- placing each person on weekly lockdown after they were potentially exposed during their active week. Hence, we effectively isolate not just the sick, but also the majority of the pre-symptomatic carriers," says Barzel.
To examine this Barzel and team simulated the spread of COVID-19 using the SEIR model, assuming that only the invisible spreaders contribute to the infection. This model tracks the number of individuals as they transition between the different stages of the disease: Susceptible (those available to contract the disease), Exposed (those who are at their pre-symptomatic stage), Infected (those who develop symptoms), and Recovered (those who are already immune).They found that employing their strategy significantly reduces the spread and helps flatten the curve.
"We can achieve more if we also adopt responsible behavior. Specifically, we expect that even during their active week, people will continue to interact with caution: avoid physical contact, increase personal space and practice hygienic behavior. These additional measures, when coupled with our alternating lockdown strategy, can help inhibit the spread, allowing us to overcome COVID-19 with reasonable economic consequences," he says.
Sustaining restrictive measures such as lockdowns is always a challenge, since some percentage of the population may not cooperate, especially in democratic and individualistic societies. The researchers therefore allowed for a fraction of defectors who continue to be active even during their lockdown phase in their simulation. This can also represent exempt individuals who hold essential positions and cannot be under lockdown. They found that even under 30% of non-cooperators would still overcome the viral spread.
The proposed weekly succession is aimed to sustain a functional economy in these challenging times. The researchers believe that, providing an outlet for people to continue their social and professional activity, at least at 50% capacity, will, in and of itself encourage cooperation, as it relaxes some of the individual stress endured under lockdown.
Barzel and team's mathematical analysis and numerical results, including all their codes, was recently published on the Arxive. Since publication the strategy has gained traction. Barzel and team have been approached by the Israel National Security Council and have presented the strategy to the academic committee advising the government. They were also contacted by international groups in the US advising 17 governments around the world. Barzel believes there is a good chance that his team's suggestion will be implemented, most probably together with complementary mitigation measures.