Delaware, Spring 2020: Cases Down 82% After Masks and Contact Tracing Joined the Stay-at-Home Order

Delaware reported its first COVID-19 case on March 11, 2020. Over the next two months the state stacked up four measures, and a CDC report tracked what happened to cases, hospitalizations and deaths as each was added.

The curve

Weekly rates climbed after the first case and the stay-at-home order, peaking in the week of April 13 at 15.0 cases, 2.0 hospitalizations and 0.8 deaths per 10,000 people. They then fell (cases 18%, hospitalizations 20%, deaths 13%), rose slightly in the week of April 20, and dropped again the week the mask mandate took effect. From late April through June, with contact tracing added, incidence fell 82%, hospitalizations 88% and deaths 100%.

Weekly confirmed COVID-19 cases, hospitalizations and deaths in Delaware, with mitigation measures marked, March 9 – June 15, 2020. CDC.

The stay-at-home order and case investigations started weeks before the peak and likely contributed to the decline — the effect was not immediate, because new cases reflect exposures from earlier weeks. The steep further drop after the mask mandate fits what masks do: stop respiratory droplets carrying the virus from reaching other people, whether the wearer has symptoms or not.

Case investigation, by the numbers

From March 11 to June 25, 9,762 confirmed cases were reported. 6,527 (67%) patients were interviewed and asked to self-isolate, and 5,823 of them had been released from isolation when the data were collected. Their median age was 41, and 55% were female; 88% reported symptoms, and 55% reported close contact with someone diagnosed.

Those not reached had not answered calls (12%), were in a hospital or long-term care facility (8%), had no phone number (7%), had died (4%), or other reasons (2%).

Where contact tracing fell short

Among the 882 contacts reached — median age 25, and 87% living with someone who had COVID-19 — 161 developed symptoms in quarantine and were urged to get tested. A manual check found that only 20 were tested, and 4 were diagnosed; with resources stretched, there was no active follow-up. Contact tracing began only in May, when the Delaware National Guard was activated to help.

Starting interviews sooner might improve recall and find contacts earlier; daily and weekly monitoring of "other" outcomes, and active follow-up and referral for testing, could reach more people.

What it shows, and what it can't

The authors' conclusion: no single measure is likely to work alone, but stay-at-home orders and public mask mandates, combined with case investigation, contact tracing and prompt isolation and quarantine, can cut new cases, hospitalizations and deaths. The study could not say how much each measure contributed; adherence to the stay-at-home order and to masking wasn't measured, and quarantine adherence was self-reported.

Sources

Based on Kanu FA, Smith EE, Offutt-Powell T, Hong R, Delaware Case Investigation and Contact Tracing Teams, Dinh TH, Pevzner E, "Declines in SARS-CoV-2 Transmission, Hospitalizations, and Mortality After Implementation of Mitigation Measures — Delaware, March–June 2020," MMWR Morbidity and Mortality Weekly Report volume 69, number 45, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The imported copy was cut short; the missing text is from the report's full text in PubMed Central (PMC7660664). The report gives two different descriptions of the same 721 contacts, so that figure is left out.