Images of a nearly empty Times Square and a cleared-out Grand Central Terminal signaled the stillness of New York City under shutdown orders this spring — orders that led to about a 70 percent reduction in the spread of the novel coronavirus, according to a new study.

A forthcoming study from Columbia University’s Mailman School of Public Health and the New York City Department of Health and Mental Hygiene has yet to be peer reviewed, which means other scientists have not had a chance to cast a critical eye on the research. But the authors conclude that public health interventions meant to limit contact between people, such as closing schools and telling nonessential workers to stay home, “likely contributed to the largest reduction in transmission in the population overall.”

The city began closing public schools the week of March 15 and imposed stay-at-home orders for everyone except essential workers the following week. Restrictions remained in place until June, when the city began gradually reopening while keeping indoor dining and other high-risk activities off-limits.

Amesh Adalja, an infectious-disease physician and senior scholar at the Johns Hopkins Center for Health Security, said it is not surprising that shutdowns would curb the spread of the coronavirus.

But it is critical to also look at the “whole picture,” said Adalja, who was not involved in the study. “What are the impacts on other health measures and the overall well-being of a population?”

He cited, for example, the impact that restrictions may have had on the ability for psychiatrically ill patients to go to group therapy, or the impact on measles vaccination rates.

“The evidence of its ability to decrease covid-19 cases is not evidence for it being a go-to tool,” Adalja said, adding: “It’s just a blunt tool that can only be used for a very short period of time in dire circumstances. You end up causing a lot of collateral issues that you’ll have to address down the road. The key is to think about public health measures in the long range. What is sustainable?”

Masks also played a big role in curbing the spread, the researchers found.

The forthcoming study found that the widespread use of face coverings was linked to a 7 percent reduction in transmission during the first month the mandate was implemented in public spaces.

“But that effectiveness varied very substantially across different age segments of the population,” said Wan Yang, assistant professor of epidemiology at the Columbia Mailman School.

Face coverings helped reduce transmission by about 20 percent among people 65 and older, compared with less than 10 percent for most other age groups.

Those numbers reflect the reality that people do not always wear their masks consistently or correctly.

Yang, the study’s lead author, said it is not a surprise that older populations had more effective mask behavior, compared with younger people who may choose comfort over mask compliance.

“The elderly, they know they’re at [higher] risk, so they’re more willing to wear face masks correctly when they’re outside,” she said, adding: “Just from personal observation, they tend to be more cautious. Sometimes I saw some elderly people wear two masks, just to cover all the bases.”

If other age groups could model the mask-wearing behavior of older adults, universal face-covering could reduce virus transmission by up to 32 percent, the researchers wrote. There is “definitely room for improvement” in mask-wearing, Yang said.

Improving mask-wearing will be key as the city continues to reopen and as more people venture outdoors after months of staying home, Yang said, especially for “reducing the risk of another resurgence of covid for places that were able to get it under control and are now reopening, trying to gain some normalcy after a severe period of the pandemic.”

Jeffrey Shaman, a professor of environmental health sciences at Columbia and a co-author on the study, said in a statement, “It’s crucial that we find ways to boost consistent and correct mask use in settings where social distancing is not possible.”

The researchers used city data on case numbers and deaths, as well as mobility data from SafeGraph, a company that aggregates cellphone location information, to simulate the spread of the coronavirus and estimate transmission. The study was posted on MedRxiv, a preprint server for research that has not been peer-reviewed.

The findings are especially notable as the state of New York, once the U.S. epicenter of the coronavirus, moves ahead with its reopening stages. All the state’s regions have entered Phase 4 of reopening. For the first time in months, New York City diners will be able to eat inside restaurants as of the end of September.

The new study on New York’s shutdown measures also comes as a federal judge ruled this week that restrictions ordered by Pennsylvania Gov. Tom Wolf (D) to curb the spread of the coronavirus were unconstitutional. President Trump, who had repeatedly chided Democratic governors for statewide restrictions, applauded the decision and said he hopes it is followed by similar decisions in other states.

Adalja said even when they were implemented in New York, stay-at-home orders were a “last resort because so many mistakes were made in January and February and March. Hospitals were overrun. We had no ability to know who was infected and who wasn’t.”

Yang said research has shown that shutdowns work to limit viral spread, even if they are an undesirable strategy.

“No one wants a lockdown, right? So when it’s implemented, it’s really critical,” she said. “Our studies show that it is very effective, especially for covid. It’s really hard to detect. When you have a substantially high infection rate detected in a population, chances are we already have widespread community transmission.”

Looking ahead, Yang said that in addition to improved mask-wearing, public health protocols including contact tracing, limiting occupancy in businesses, and testing and isolation will all be needed to simultaneously help curb the risk of a resurgence.

Public messaging, Yang said, including about the importance of face coverings, is critical.

“What can you do to prevent another lockdown? We’re all in the same boat, so we should communicate that everyone’s efforts count,” she said. “This is not about individuals. In order for it to work, we as a society have to act collectively.”