Harvard: On-off social distancing to be needed until 2022
it may be necessary to keep having periodic intervals of quarantine until the US healthcare system is fortified.Europost
A one-time lockdown won't halt the novel coronavirus, Harvard scientists who modeled the pandemic's trajectory said Tuesday. Instead, it can turn out to be counter-productive and repeated periods of social distancing may be required into 2022, to prevent hospitals from being overwhelmed. Their study comes as the US enters the peak of its COVID-19 caseload and states eye an eventual easing of tough lockdown measures.
The Harvard team's computer simulation, which was published in a paper in the journal Science, assumed that COVID-19 will become seasonal, like closely related coronaviruses that cause the common cold, with higher transmission rates in colder months. But much remains unknown, including the level of immunity acquired by the previous infection and how long it lasts, the authors said.
"We found that one-time social distancing measures are likely to be insufficient to maintain the incidence of SARS-CoV-2 within the limits of critical care capacity in the United States," lead author Stephen Kissler said in a call with reporters.
"What seems to be necessary in the absence of other sorts of treatments are intermittent social distancing periods," he added.
Conversely, too much social distancing without respite can be a bad thing. Under one modeled scenario "the social distancing was so effective that virtually no population immunity was built and there was a resurgence of infection when the simulated social distancing measures were lifted," the paper said. Hence, 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.
In this regard, The Hardvard medical researchers present two possible futures. 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.
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. But in order to determine when the thresholds to re-trigger distancing are crossed, widespread viral testing would be required, said the authors. The duration and intensity of lockdowns can also be relaxed as treatments and vaccines become available.
"Widespread surveillance will be required to time the distancing measures correctly and avoid overshooting critical care capacity," the authors observe.
The authors, however, acknowledged a major drawback in their model is how little we currently know about how strong a previously infected person's immunity is and how long it lasts. At present, the best guesses based on closely-related coronaviruses are that it will confer some immunity, for up to about a year. There might also be some cross-protective immunity against COVID-19 if a person is infected by a common cold-causing betacoronavirus.
Nevertheless, one thing is almost certain: the virus is here to stay. The team said it was highly unlikely that immunity will be strong enough and last long enough that COVID-19 will die out after an initial wave, as was the case with the SARS outbreak of 2002-2003.
Outside experts praised the paper even as they emphasized how much remained unknown.
"This is an excellent study that uses mathematical models to explore the dynamics of COVID-19 over a period of several years, in contrast to previously published studies that have focused on the coming weeks or months," Mark Woolhouse, an infectious disease epidemiologist at the University of Edinburgh said.
"It is important to recognize that it is a model; it is consistent with current data but is nonetheless based on a series of assumptions - for example about acquired immunity - that are yet to be confirmed."