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Social Distancing and its Effect on Reducing the Spread of Illness

We are seeing increasing evidence that social distancing policies enacted at the state level are causing decreases in viral transmission. This analysis is based on our real-time illness signal collected over the last two weeks which is highly correlated with the national influenza-like illness (ILI) reported by the Center for Disease Control (CDC). Our illness signal is updated daily1.

As a starting point for this analysis, we looked at state-level policies because of the challenges associated with tracking inconsistent and rapidly changing policy measures across overlapping jurisdictions. We looked at many of the common interventions, which are summarized below and broken down by state and date of adoption.

Milestone WA OR CA NY SC MI FL
State of Emergency Declaration 2/29/20 3/8/20 3/4/20 3/7/20 3/13/20 3/10/20 3/1/20
Restriction on Public Gatherings 3/11/20 3/12/20 3/12/20 3/12/20 3/17/20 3/13/20
Restaurants Closed for Dine-In 3/15/20 3/16/20 3/18/20 3/16/20 3/17/20 3/16/20 3/20/20
Schools Closed 3/17/20 3/16/20 3/16/20 3/18/20 3/13/20 3/16/20 3/16/20

Before looking at individual state results, it’s important to include a caveat in regards to the findings presented here. In particular, what we are seeing here is likely an overwhelming decrease in seasonal cold and flu transmission rather than any effect specific to COVID-19. This conclusion is driven by two factors:

  1. We assume that the incidence of seasonal cold and flu viruses driving infections currently is significantly higher than COVID-19.
  2. The median incubation period (time between infection and symptom presentation) for the flu is 3 days, whereas for COVID-19, the current estimates for incubation are closer to 5 to 6 days2. Therefore, following the introduction of social distancing measures, we would first expect flu infections to drop.

COVID-19 and the flu are both respiratory viruses that are transmitted primarily by respiratory droplets. We interpret these declines in presumed flu infections to be a promising indication that social distancing measures will be effective at slowing the spread of COVID-19. 

The following section compares roughly four weeks of influenza-like illness levels for three states — Florida, Washington and California — that first reported COVID-19 infections. These states provide a useful case study. Washington state and California have aggressively instituted social distancing measures and have shown declines in influenza-like illness in the subsequent days and weeks. In Florida, the limited implementation of social distancing measures are associated with a prolonged and sustained increase in illness levels.

Washington

The first confirmed COVID-19 case in the U.S. was in Washington state on Jan 21. The first COVID-19 related death was in Washington state on Feb. 29. Washington was also the first state to implement widespread social distancing policies. By the first week of February, many large employers in the Seattle area had recommended or mandated work-from-home policies and the University of Washington had canceled in-person classes. By March 11 the state had already begun to ban public gatherings and by March 15, Washington had closed all restaurants and banned public gatherings larger than 50 people.

 

Washington - Share of Population with Flu-Like Illness

 

King County Atypical Illness

Atypical Illness Levels for King County, WA and surrounding areas. As of March 24, 2020.

 

California

The third U.S. case of COVID-19 was confirmed in California on Jan 25. After the first death attributable to coronavirus in California on March 4, the governor declared a state of emergency. Public gatherings larger than 1,000 people were banned in Santa Clara County on March 9, followed shortly thereafter by a statewide ban on gatherings larger than 250 people on March 12. On March 17, many counties in the Bay Area issued shelter-in-place orders, with a statewide stay-at-home order issued March 19.

 

California - Share of Population with Flu-Like Illness

Los Angeles County Atypical Illness

Atypical Illness Levels for Los Angeles County, CA and surrounding areas. As of March 24, 2020.

Santa Clara County Atypical Illness 03242020

Atypical Illness Levels for Santa Clara County, CA and surrounding areas. As of March 24, 2020.

 

Florida

On March 1, Florida reported its first confirmed case of COVID-19. Although the governor declared a state of emergency the same day, there were no mandatory restrictions implemented at the state level until March 17, when all bars and nightclubs were ordered closed. Public universities started canceling in-person classes on March 17 and 18 and public beaches as well as restaurants and gyms were ordered closed on March 20.

 

Florida - Share of Population with Flu-Like Illness

Hillsborough County Atypical Illness

Atypical Illness Levels for Hillsborough County, FL and surrounding areas. As of March 24, 2020.

Miami-Dade County Atypical Illness

Atypical Illness Levels for Miami-Dade County, FL and surrounding areas. As of March 24, 2020.

 

Through the evidence shown above for Washington state, California and Florida, strict social distancing measures appear to have an effect on reducing the total influenza-like illness. For more information about our data and methods, please check out our technical documentation and FAQ page at healthweather.us


1Miller AC, Singh I, Koehler E, Polgreen PM. A Smartphone-Driven Thermometer Application for Real-time Population- and Individual-Level Influenza Surveillance. Clin Infect Dis. 2018;67:388–97. https://doi.org/10.1093/cid/ciy073
2https://www.who.int/news-room/q-a-detail/q-a-similarities-and-differences-covid-19-and-influenza