Work posted on the medRxive pre-print server Tuesday by a group of researchers at Harvard's T.H. Chan School of Public Health in Boston suggests that multiple targeted periods of "social distancing" of various kinds will likely be necessary for the US before any vaccine is found for the disease.
There's a tension in fighting COVID-19: So-called herd immunity needs to be built up, which requires that the disease be allowed to spread to some extent, for without exposure, that immunity will never be built. But the disease must not spread so much that it overwhelms the US's medical resources.
In "
Social distancing strategies for curbing the COVID-19 epidemic," authors Stephen Iissler, Christine Tedijanto, Marc Lipsitch, and Yonatan Grad of the Chan School write that "a single period of social distancing will not be sufficient."
Paradoxically, going into an intense quarantine with nothing to follow it can actually be counter-productive.
Without repeated intervals of distancing, "there was a resurgence of infection when the simulated social distancing measures were lifted" in the model scenarios they ran.
The authors found that a resurgence could happen even after especially arduous periods of distancing, such as a 20-week period of social distancing. "The social distancing is so effective that virtually no population immunity is built."
Also: We need a Big Data effort to find a COVID-19 cure, says pioneering geneticist
Instead, the authors argue interventions need to be made multiple times over a period of time, called "intermittent distancing," at intervals that depend upon the state of the health care infrastructure at any moment in time, meaning, how much load it can absorb of critical care cases of the disease.
"Intermittent social distancing can maintain critical care demand within current thresholds," they advise.
Two possible futures according to Harvard medical researchers. In one instance, low healthcare resources mean intermittent social distancing needs to be extended out into 2022, top, while in a scenario with higher healthcare resources, distancing can end midway through 2021.
Kissler et al.
The authors suggest a threshold to be maintained is no more than 37.5 cases of the disease per 10,000 adult people in the population. That should be the "on" switch to re-commence social distancing, they argue. That threshold, they estimate, would keep the number of patients needing critical care at 0.89 persons for every 10,000 people in the population, which should be adequate to not overwhelm the health care system.
The predictions made by the researchers are constructed via a familiar, long-standing epidemiological model known as "SEIR," an acronym that stands for the "susceptible," "exposed," "infectious," and "recovered" individuals in a community. The approach uses differential equations to compute how fast a disease may spread based on how many people are in a community and how many are already sick or who have gotten better or died. It's a statistical technique, and so it's important to remember that it's not a guarantee of future trends, it's a way to model what might happen. Bear in mind, the paper is not yet peer-reviewed, and so fellow researchers have not yet vetted the work.
