Alone among major U.S. cities, Philadelphia will again require masks indoors at all restaurants, shops, offices, and other indoor public spaces next week, the city health department announced Monday. Businesses can opt to require proof of vaccination instead of requiring masks.

The mandate takes effect April 18, city Health Commissioner Cheryl Bettigole said, to give businesses time to adjust, though she encouraged Philadelphians to start wearing masks indoors in public spaces immediately. Philadelphia is the first large American city to respond to rising COVID-19 cases in recent weeks by reintroducing an indoor mask mandate.

“I sincerely wish we didn’t have to do this again,” Bettigole said. “I wish this pandemic was over just as much as any of you.”

Philadelphia established a benchmark system in February that uses case counts, hospitalizations, and the rate of case increases to determine which safety strategies are needed. The seven-day daily average of cases, 142 as of April 8, and a 60% increase in case counts over the past 10 days met the standards to reintroduce the indoor mask mandate. There were 44 people with COVID hospitalized in the city Monday, a slight decrease from last week. The city averaged 2 deaths a day over the past week, according to the New York Times COVID data tracker, a 43% decline over two weeks.

Nationally, cases are up about 3% over the past two weeks, according to the New York Times COVID data tracker. Hospitalizations are down 18% over the same time. Other Mid-Atlantic cities are seeing case increases as well, though so far there have not been reports of mask requirements resuming there. Philadelphia, which has not required indoor masking since March 2, appears to be alone among major U.S. cities in bringing the restriction back. No states currently have indoor masking rules, except for settings such as health care facilities. Federal law requires masking on public transportation. The Centers for Disease Control and Prevention does not recommend masks for areas with Philadelphia’s current COVID conditions, describing the risk level as low, and PolicyLab at Children’s Hospital of Philadelphia also came out against bringing back indoor masks.

The city health department has the flexibility to react to local conditions, Bettigole said, and among the concerning signs the department considered was the recent spread of the BA.2 subvariant in the United Kingdom, where it caused an increase in hospitalizations and deaths.

» READ MORE: Philly's new COVID system makes it hard to stay unmasked. We crunched the numbers.

While the local health care system has not reported being stressed by COVID recently, Bettigole said hospital capacity isn’t the only concern as cases rise. The impact on people particularly vulnerable to COVID is another significant factor, she said.

Businesses, restaurants, and offices don’t have to enforce the mask mandate if they require proof of vaccination from customers and employees, said James Garrow, a spokesperson for the health department.

The Philadelphia School District, which had lifted its mask mandate in March, had already planned a one-week return to masking when students and staff return from spring break on Monday. They will now simply remain in masks as long as the city’s mask mandate remains in place, a schools spokesperson said.

» READ MORE: The return of masks gets eye rolls from some, support from others

The restaurant and hospitality industry, which faces competition in the Pennsylvania suburbs where no mandates are in effect, expressed frustration.

“This announcement is a major blow to thousands of small businesses and other operators in the city who were hoping this spring would be the start of recovery,” said Ben Fileccia, senior director of operations at the Pennsylvania Restaurant and Lodging Association.

BA.2 surging

The latest surge appears to be caused by the spread of the BA.2 subvariant of omicron. This iteration of COVID seems no more likely to cause serious illness than the original omicron, health experts said, but is slightly more transmissible. Present in the Philadelphia area since early January, the subvariant is becoming increasingly common. The Penn Center for Research on Coronavirus and Other Emerging Pathogens, which conducts genetic sequencing on samples taken through testing, found more than 75% of the COVID cases in the area last week were caused by BA.2.

“If we fail to act now, knowing that every previous wave of infections has been followed by a wave of hospitalizations and a wave of deaths, it’ll be too late for many of our residents,” Bettigole said.

She noted that 750 Philadelphians died in three months during the winter case surge driven by omicron, which spread ferociously but generally caused less serious illness than the delta variant.

”We don’t know if the BA.2 variant in Philadelphia will have the kind of impact on hospitalizations and deaths that we saw with the original omicron variant this winter,” Bettigole said. “I suspect that this wave will be smaller than the one we saw in January.”

When cases increase, hospitalizations typically follow suit a week or two later. Health officials aren’t sure if the BA.2 subvariant will cause the same pattern, though. The large number of people who are vaccinated — about 76% of city residents 12 and older have completed the initial series — or are immune through prior COVID infections could keep hospitalizations from increasing along with cases.

» READ MORE: Is the latest COVID-19 surge in Europe coming to the U.S.?

”This is our chance to get ahead of the pandemic,” Bettigole said, “to put our masks on until we have more information on the severity of this variant.”

Transparency and consistency

Public health experts credited the city for establishing a benchmark system to dictate when safety restrictions would increase, and sticking to them.

“They set out clear metrics and these are the metrics,” said Jennifer Kolker, associate dean for public health practice at Drexel University. “To me, there weren’t a whole lot of decisions to be made.”

City health officials have acknowledged that the publicly available case counts used in determining safety interventions are increasingly unreliable due to home COVID tests, which largely aren’t reported publicly. Most likely, said Garrow, the data is likely an undercount of the real prevalence of COVID in Philadelphia.

“A lot of people are taking the home antigen tests, and unless they have really bad symptoms they’re probably not even going to call a doctor,” said Thersa Sweet, associate professor in Drexel’s department of epidemiology and biostatistics.

As to whether masks are needed, experts said it is difficult to say with certainty. Sweet noted that many people remain vulnerable to COVID even after vaccination, like the immunocompromised or people with underlying conditions such as diabetes who would benefit from the added protection as cases increase.

Kolker felt there was little harm in requiring masks again. Even cases that don’t require hospitalization can cause people to become very ill, and the risk of long COVID — meaning symptoms that linger for longer than a month — remains poorly understood, even though vaccination appears to reduce the likelihood of it developing.

“We want to keep kids in schools, and we want to keep workers going, and we want to keep flights running,” she said. “There are ripple effects beyond just health because it’s a really disruptive thing. Just transmission is super disruptive.”

Neal Goldstein, an epidemiologist at Drexel’s Dornsife School of Public Health, was less certain masks are needed now, given that serious cases remain much lower than during the surge.

“It suggests that even though we are having the uptick in cases,” he said, “fortunately we are not seeing the downside of this in terms of hospitalizations or mortality.”

He acknowledged, though, it was difficult to predict the results if the city didn’t require masks.

As Bettigole said, “We have yet to see a wave of COVID 19 infection that did not result in an increase of hospitalizations.”

A blow to businesses

Reimposing the mask mandate is, “definitely a devastating blow for a lot of businesses,” said Jabari Jones, president of the West Philadelphia Corridor Collaborative.

The mandate has many business owners “on edge,” he said, not knowing if the restrictions will last through the summer.

“It’s deflating to hear that the city plans to bring back the mask mandate,” Fileccia said, “especially without having conversations with anyone from the industry about timing and approach.”

The health department developed the alert system in consultation with businesses, Bettigole said, done in part so they could anticipate changes in safety precautions.

“This is what we had discussed with them,” she said, “and so there absolutely was stakeholder input in the creation of these response levels.”

Tara Alexander, owner of plant shop Urban Jungle on East Passyunk Avenue, hasn’t been requiring masks, but has noticed more customers seemed to be coming in with them on in recent days.

“It’s a bummer,” she said of the mandate, “but as long as we’re all keeping each other safe I guess it’s a good thing.”

Staff writers Erin McCarthy, Justine McDaniel, Kristen Graham, and John Duchneskie contributed to this story.