Lockdown Led to 70% Drop in NYC COVID Spread, Masks Also Effective

September 14, 2020

Reducing contact rates—mainly via school closures and voluntary or mandated stay-at-home measures—contributed to around a 70 percent reduction in the transmission of COVID-19 in New York City during the spring pandemic wave from March to the June reopening. Widespread use of face coverings contributed an additional 7 percent reduction, and up to 20 percent reduction among those aged 65 and older during the first month face covering was mandated in public places.

The study by scientists at Columbia University Mailman School of Public Health and the New York City Department of Health and Mental Hygiene appears ahead of peer review in the preprint server medRxiv.

The new study is in line with previous modeling studies estimating that lockdowns reduced COVID-19 transmission by 58 percent in Wuhan, China, 45 percent in Italy, and 77 in France.

The New York City study finds reducing contact rates was highly effective for most age groups—contributing to at least a 50 percent reduction in transmission across age groups, ranging from a 51 percent reduction among 1-4 year-olds to 83 percent among 5-14 year-olds.

Face Coverings Are Key to a Safer Reopening

If the city maintained the same usage rates of face coverings in April into the reopening period starting June 8, the researchers estimate that face coverings would reduce overall COVID-19 transmission by approximately 9–11 percent during reopening when people spend a greater amount of time outside homes and use face coverings outside. However, if everyone wore masks in the same degree and manner as older adults, the city could reduce overall transmission by as much as 28–32 percent.

The researchers note their estimates differ from those measuring how effective face coverings can prevent COVID-19 infection when used correctly. Rather, estimates reflect the effectiveness of face coverings and the impact of this public health intervention in reducing transmission at the population level, which also depends on the usage rate of face coverings, whether face coverings are properly used, the amount of time face coverings are used, in addition to mask efficacy. Observational studies and surveys report about two-fold high rates of face covering usage among older adults versus younger adults and minors. The researchers estimate much higher effectiveness among older adults (~20% for both 65-74 and 75+ year-olds vs. <10% for other age groups).

“Improving effective use of face coverings, especially among younger people, would significantly mitigate the risk of a resurgence in COVID-19 infections during re-opening,” says senior author Jeffrey Shaman, PhD, professor of environmental health sciences at Columbia Mailman School. “It’s crucial that we find ways to boost consistent and correct mask use in settings where social distancing is not possible.”

“Overall, our study supports the need for multiple interventions, including restricting occupancy to reduce contact rates, universal face covering, testing and contact tracing, and isolation and timing treatment of active infections,” says lead author Wan Yang, PhD, assistant professor of epidemiology at Columbia Mailman School. “We need to implement all of those simultaneously in order to effectively mitigate the spread of COVID-19.”

Study Methods

The researchers developed a model to simulate the spread of COVID-19 in New York City using city data on cases, deaths, along with mobility data to estimate transmission. They first used mobility data as a proxy for population contact rates to estimate the transmission reduction associated with interventions reducing contact rates, including school closures and the stay-at-home mandate. They then further separated this impact from the overall transmission reduction to estimate the impact of face coverings. The researchers also validated their model estimates based on the accuracy predicting cases and deaths eight weeks beyond the study period.

Background on COVID in New York City

New York City experienced widespread COVID-19 transmission citywide since early March and recorded over 200,000 cases and over 21,000 COVID-19 confirmed or probable deaths during the following three months. New York State and New York City implemented multiple intervention measures, including health promotion campaigns in early March, telecommuting and staggered work schedule recommendations beginning the week of March 8, public schools closure starting the week of March 15, stay-at-home orders for non-essential workers starting the week of March 22, and requirements for use of face covering in public starting the week of April 12. With these overlapping and far reaching public health interventions, case diagnoses and hospitalizations peaked in April, and started to decline substantially in late April and May. The city began its phased re-opening of industries starting the week of June 7.

Co-authors included Wan Yang and Jeffrey Shaman from Columbia Mailman School, and Jaimie Shaff from the New York City Department of Health and Mental Hygiene (NYC DOHMH).

This study was supported by the National Institute of Allergy and Infectious Diseases (AI145883), the National Science Foundation Rapid Response Research Program (RAPID; 2027369), and the NYC DOHMH.

Shaman and Columbia University disclose partial ownership of SK Analytics, a provider of influenza forecasting and analytics services. Shaman discloses consulting for Business Networking International. Other authors have nothing to disclose.