The Science of Social Distancing

April 9, 2020

News is generated quickly and policies are constantly in flux as humanity does its best to keep up with SARS-CoV-2. As a society, we've become oddly familiar with epidemiological terms like "social distancing" and "quarantine." We've seen models and heard public messaging about the importance of "flattening the curve." But many questions still remain. Everyone wants to know will social distancing slow the spread of COVID-19 and when will it end?

At a base level, we're all craving some background knowledge about the science of social distancing. The good news? Although it may seem like we're living in unprecedented times, there's more scientific data and historical context to Non-Pharmaceutical Interventions (NPI) than you might think.

For centuries, people have used containment strategies (quarantine, border controls, contact tracing and surveillance) to limit the spread of communicable disease. An organized community response to infection is most critical in the absence of pharmaceutical treatments and cures, but varying levels of political, ethical and socioeconomic controversy have long accompanied these practices. Management of NPI requires a careful balance between public and individual welfare.

Will Social Distancing Slow the Spread of COVID-19?

Historical Context

We can learn a lot about our current state of affairs by reexamining 2 of the most devastating pandemics in recorded history - the Black Plague and the 1918 Flu.

The term "quarantine" is derived from the Italian number "quaranta," or 40, and the practice originated in 1347-1348, when the "Black Plague" was sweeping Europe. In an attempt to slow the spread of disease, Venice founded the first quarantine island, Lazzaretto Vecchio, Santa Maria di Nazareth Island, and became the first city to close its ports to incoming ships. When a successive wave of the plague hit in 1485, the same Italian city required all vessels coming from infected ports to be detained for 40 days. Unfortunately, it's impossible to know how many deaths were prevented by improved sanitation efforts and decreased person-to-person contact during the plague, but historical accounts like these illuminate the longevity of NPI practices.

The 1918 Flu was the most deadly pandemic in human history. It is also one of the largest examples of NPI implementation on record. From September 1, 1918 - April 5, 1919, the world mourned the loss of more than 50 million lives as a result of the flu. Approximately 500,000 of those flu-related deaths occurred in the United States. Data from a collaborative study between the University of Michigan Medical School and the CDC show that U.S. cities that applied early, sustained and layered NPI, including quarantine, self-isolation, school closures and social distancing, experienced the greatest reductions in weekly death rates from September 8, 1918 - February 22, 1919. Furthermore, many cities that deactivated NPI measures early experienced 2 peaks in mortality. However, none of these cities experienced their second mortality peak while the first set of NPIs was still active. This data highlights the transient protective nature of NPI and further suggests a causal relationship between NPI and decreased disease mortality.

In other words, social distancing and self-isolating measures must be activated as close to the time of disease onset as possible and sustained throughout the peak of disease trajectory in local communities in order to be most effective. If executed properly, non-pharmaceutical interventions can help slow the spread and decrease the number of disease-related deaths during pandemics.

In recent years, NPI measures have been influential in lessoning the impact of H1N1 and containing the 2003 outbreak of SARS-CoV.

Scientific Overview

COVID19 social distancing sign on the floor of a supermarket.
COVID19 social distancing sign on the floor of a supermarket.
Source: Brandon Gregory

It's important to recognize that NPI is unlikely to change a population's susceptibility to SARS-CoV-2, or any other microbe, for that matter. That means it's not a long-term solution to the underlying problem. But it can help. Social distancing can prevent the spread of communicable disease. We know that SARS-CoV-2 spreads rapidly through person-to-person contact. Therefore, limiting human contact will limit the spread of the virus. The primary goals of NPI when it comes to COVID-19 are to alleviate overcrowding in hospitals and borrow time for scientists to develop vaccine and drug candidates.

How Long Will Social Distancing Last?

If we recognize that NPI has the potential to help but not fix the problem, the question still remains, how long should social distancing practices and stay-at-home orders remain in place? Unfortunately, the answer is not very straightforward.

Three Things that Complicate COVID-19 Modeling

  1. Extended incubation period. 

Determining when disease incidence will peak is complicated by the extended incubation period of SARS-CoV-2. Symptoms of COVID-19 can take 2-14 or more days to develop, and severe cases often don't become critical until 3-4 weeks after onset. In contrast, symptoms of the flu generally develop within 1-4 days, and the incubation period for SARS-CoV is 2-7 days. That means there's a "stealth phase" that occurs between the onset of COVID-19 and the peak of disease trajectory. And because it takes so long for patients to show signs of illness, they are more likely to spread the virus before they even know they've been infected. All of these things add to the challenge of SARS-CoV-2 containment.

  1. Unanswered questions about immunity.

Because this is a novel virus, and we're in the first season of infection, we don't yet have enough data to determine whether exposure to SARS-CoV-2 will confer immunity to recovered patients, and we definitely don't know how long that immunity will be protective if developed.

Advancements in SARS-CoV-2 serology testing have moved us much closer to answering these questions. If we're able to identify who's immune to SARS-CoV-2, strong social distancing measures can likely be alleviated. Vulnerable populations will be better protected, and immune individuals will be able to return to work.

  1. Unknown seasonality.

As spring approaches, everyone's talking about the seasonality of the virus. How will SARS-CoV-2 respond to warmer weather? If this coronavirus behaves like other seasonal viruses, we might experience relief from its siege during the summer months and see a resurgence of infection again in the fall. If the virus has no seasonality, we will continue to experience consistent outbreaks throughout the year.

Experts agree that different social distancing strategies should be implemented in the presence or absence of seasonality of the virus. According to Dr. Marc Lipsitch, Director of the Center for Communicable Disease Dynamics, Harvard T.H. Chan School of Public Health, in the absence of seasonality, all social distancing is helpful, and long, moderate social distancing is best. If seasonality is present, long, weakly effective social distancing is better. Strong measures will only delay the recurring peak of SARS-CoV-2, and there is concern that a winter resurgence of COVID-19 would coincide with flu season.

Cycled social distancing approaches have the potential to be most beneficial regardless of seasonality. Cyclical activation and deactivation of NPI would allow for the slow accumulation of herd immunity while simultaneously reducing some of the socioeconomic burdens of living in a locked down, isolated society.

Socioeconomic Impact

We must not overlook the hardships that are experienced at community and individual levels as a result of sustained social distancing efforts. Economies suffer when restaurants and businesses collectively close. People lose their jobs and income. Social isolation and economic pressures can result in significant deterioration of mental and physical health. Still, in the absence of a vaccine and therapeutics, social distancing is the only mechanism we have in place to slow the spread and death rates of COVID-19. That means the historical context, scientific evidence and socioeconomic impact of social distancing must all be taken into account as we chart our way forward as a nation forced to cohabitate (for now) with SARS-CoV-2.

Author: Ashley Hagen, M.S.

Ashley Hagen, M.S.
Ashley Hagen, M.S. is the Scientific and Digital Editor for the American Society for Microbiology and host of ASM's Microbial Minutes.