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Declines in SARS-CoV-2 Transmission, Hospitalizations, and Mortality After Implementation of Mitigation Measures— Delaware, March–June 2020

Our take —

Following the implementation of multiple mitigation measures, specifically, the introduction of a statewide mask mandate on April 28, COVID-19 incidence, hospitalizations, and deaths per capita declined significantly through June 2020 in Delaware, US. The independent contribution of each of these mitigation measures to observed reductions in COVID-19 transmission and mortality, however, could not be determined.

Study design


Study population and setting

Investigators aggregated laboratory-confirmed and probable COVID-19 cases in Delaware from March 11 to June 25, 2020. Four public health mitigation measures were implemented during this timeframe: case investigations (March 11), statewide shelter-in-place ordinance (March 24 to June 1), statewide mask ordinance (April 28), and contact tracing (May 12). Investigators assessed changes in COVID-19 incidence, hospitalizations, and deaths (per 10,000 Delaware residents per week, respectively) over the same period that these mitigation measures were introduced. Contact tracing outcomes were also described for all laboratory-confirmed COVID-19 cases.

Summary of Main Findings

A total of 9,762 COVID-19 cases were identified between March 11 and June 25, 2020. COVID-19 cases, hospitalizations, and deaths per capita increased throughout March, peaking in mid-April. Following the introduction of a statewide mask mandate on April 28, COVID-19 incidence, hospitalizations, and deaths declined at a rate of 82%, 88%, and 100%, respectively, through June. Among laboratory-confirmed COVID-19 cases, 67% were successfully contacted by health department staff and provided isolation instructions, of whom 83% either refused to name any close contacts or could not recall contacts. Among cases who named close contacts (mean = 2.5 close contacts), contact information was obtained from 66% of the 2,834 contacts identified, 47% of whom were successfully contacted and provided quarantine instructions. The median time from symptom onset in the case to contact elicitation was 8 days and from contact elicitation to contact interview was 2 days.

Study Strengths

Investigators assessed trends in COVID-19 incidence as well as hospitalizations and deaths throughout a period when multiple public health mitigation measures were enacted in Delaware.


Given the absence of suitable comparison groups in analysis, observed reductions in COVID-19 incidence, hospitalizations, and deaths cannot be exclusively attributed to the implementation of public health control measures. Furthermore, this study did not include any individual-level behavioral measures on compliance or adherence to public health mitigation effort.

Value added

This is among the first studies to assess the potential effect of multiple mitigation measures on health indicators across the COVID-19 illness continuum (infection, hospitalization, and death).

This review was posted on: 14 January 2021