The latest pandemic paradox in the Philadelphia region is that the city, while less vaccinated than virtually all its neighboring counties, has a lower rate of COVID-19 cases than any of them.

This has continued even as the suburban counties outstrip Philadelphia’s vaccination rates. The city has fully vaccinated about 56% of its total population. Only Gloucester County has a lower vaccination rate. Bucks, Delaware, and Burlington Counties all have vaccination rates of nearly 65% or better. Yet COVID-19 is still infecting a larger percentage of their populations.

The reasons for the seemingly counterintuitive trend are not clear, but the Philadelphia Department of Public Health has a theory.

“The only difference I see is the mask mandate,” said Cheryl Bettigole, the city’s acting health commissioner. “It’s hard to see what the difference is otherwise.”

Philadelphia reinstated its indoor mask mandate in August as the delta variant drove a new surge in COVID-19 cases. The mandate remains in effect and the city said in early September that it had no intentions of ending it any time soon. There is no similar indoor mask mandate through the rest of the state. New Jersey recommends indoor masking but does not mandate it.

City-gathered data from early October found about 80% of people entering and exiting shops had masks on, and the majority were wearing them correctly, said James Garrow, a health department spokesperson.

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“Step on the other side of City Avenue and masks disappear, so we definitely think that Philadelphians are better masked,” he said.

During much of the spring and summer, COVID-19 case rates in the city and its surrounding Pennsylvania and New Jersey counties were largely similar. But according to The Inquirer’s tracker of case and vaccination data, that hasn’t been the case since September, when the city began consistently reporting lower case rates than its neighbors.

“I think that for all we criticize ourselves for our behavior as sports fans and the hitchhiking robot,” Bettigole said, “we actually are a city that takes care of each other.”

National trends are somewhat similar

Boston, New York, City, Chicago, and Washington D.C. all have some form of indoor mask mandates, though the policies in their surrounding counties and cities vary.

The pattern in the Philadelphia metro area mirrors trends in some, but not all, Northern and Northeastern metros. Suffolk County, Mass., which contains Boston, has both a lower vaccination rate and a lower case rate than surrounding suburban counties such as Norfolk, Essex, and Sussex. Massachusetts recommends indoor masking but does not mandate it.

In Illinois, suburban counties have higher case rates than the Chicago metro area in Cook County, although vaccination rates vary in those suburbs. Illinois has had a statewide indoor mask mandate since August.

New York City, though, lags its suburban counties in vaccination rates, but only some of its suburbs are currently posting higher COVID-19 case rates.

And in the D.C. area, the pattern is inverted: again, the city has lower vaccine uptake, but it also has higher case rates than the suburbs today.

Other potential factors

Health experts said it’s plausible that masking makes the difference, though other factors could also be in play.

“I’m inclined to think that indeed indoor masking is definitely helping in this situation,” said Thersa Sweet, associate professor in Drexel University’s department of epidemiology and biostatistics.

Other possible explanations, she said, don’t seem to match the data. The divide between city and suburban case rates could be a result of more testing in the suburban counties, but Sweet said that isn’t likely because Philadelphia’s positivity rate is so low. Natural immunity among unvaccinated people who have caught COVID-19 and recovered likely wouldn’t account for it either, she said, since that protection is less effective than vaccines offer.

Health officials elsewhere, though, say it is difficult to prove mask wearing is the difference maker, and there are many other factors that could account for the difference in infection rates.

“The many layers of mitigation measures used to control COVID-19, combined with different community settings,” Chester County’s health department said Friday in a statement, “means that it is difficult to narrow it down to just masking in schools, masking and proof of vaccination within the workplace, or in restaurants and retail outlets.”

About 61% of that county’s total population is fully vaccinated.

“In general,” said Richard Lorraine, chief medical officer in Montgomery County’s Office of Public Health, “we are cautious about drawing definitive conclusions from small differences in this data.”

That county, where almost 62% of people are fully vaccinated, has reported an average of about 18 cases per 100,000 a day over the past week, he said, compared to almost 13 per 100,000 in the city. The difference isn’t large, Lorraine said, and Philadelphia has a younger population than Montgomery County, which could play a factor in producing fewer symptomatic cases. Other relevant factors include whether people seek care in their own counties or elsewhere, how much people are traveling and moving about the region, employers’ vaccine requirements, and access to care and testing.

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“Overall, it is unlikely that mask policy accounts significantly for any current differences in COVID case rates between Philadelphia and Montgomery County,” Lorraine said.

Montgomery and Chester County officials said the counties don’t track mask-wearing habits.

“As far as I know, there has not been any direct study of masking compliance in businesses in either Philadelphia or Montgomery County, so there would be no way to judge this contribution,” Lorraine said.

The challenge throughout the pandemic, Sweet said, has been gathering detailed data on a new virus that has continued changing since it swept the world in 2020.

“What was first in the American population in the early time in March is a different virus than we got hit with this summer,” Sweet said. “I know the general public probably gets really frustrated with how long it takes to get these answers, but at the same time how do you rush these things?”

Staff Writers Justine McDaniel, Erin McCarthy, and Laura McCrystal contributed to this story.