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The cold weather coronavirus surge health experts long warned about appears to be taking off in Philadelphia. As election week gets underway, key metrics are hitting numbers not seen in the city since the summer.

Compared to September, when case counts and test positivity rate in Philly were plateauing at their lowest recorded values, the last week in October saw them nearly quadruple.

“We’re entering a difficult and dangerous period of this epidemic,” Health Commissioner Tom Farley said on Oct. 27. “Possibly the worst period of the entire epidemic.”

During the second-to-last week of September, the city was averaging 90 new cases per day. That was out of more than 3.5k tests daily, with an average positivity rate of 2.7%. During the last seven days in October for which we have full data, those numbers jumped. The city averaged 361 new cases daily out of around 4k tests, with an average positivity rate of 9.4%.

There is a bright side, which is that hospitalizations and fatalities in the city have not spiked in nearly the same fashion.

The total number of positives recorded in Philadelphia in October (6,700) is very close to that of May (7,057), but when viewed as a percentage of total positive cases, the rate of hospital admissions and deaths in Philly are both at an all-time low right now.

(Note: The figures for October are likely to rise slightly in the coming week, since there are some lags in data reporting. We’ll update this post as they do.)

Why haven’t the people testing positive suffered outcomes as severe as during the region’s previous pandemic peak?

A few possible factors likely combine to provide the answer.

1) Younger people are testing positive compared to in the spring

From the beginning, scientists and doctors found the coronavirus to be much more harmful to older adults.

In May, more than a tenth of positive cases in Philly were people 75 years of age or older, and about a third were people aged 55 and up.

By October, the 75+ crowd accounted for less than 5% of positives, with the total group 55 and older shrinking to around 20%.

2) Doctors now have experience, so treatments are getting better

There’s still no cure for COVID-19, but there’s much more shared knowledge about what has and hasn’t worked well for the millions of people who’ve contracted the disease across the U.S. and globe.

In the Philly region specifically, the Inquirer reports, there’s also “less panic” among doctors, which they cite as a significant factor.

3) Lower doses of the virus are being spread, which may result in lighter symptoms

Nothing is conclusive yet, but some recent studies suggest that how much of the coronavirus gets into your body — your viral load — can be a factor in determining how sick you get, Dr. Farley noted.

All the mask-wearing and social distancing people are doing, while not always perfectly observed, are likely making a difference, he said. So when those who have the virus unknowingly spread it to others, they could be sharing fewer particles, leading to less severe cases.

Philly’s lower hospitalization and fatality rates do not mean “relax,” Dr. Farley stressed last week. This could be just the beginning of a bigger spike — something that would be expected, he said, if SARS-CoV-2 follows the same pattern as the virus that causes influenza.

Most of the spread in Philadelphia is occurring within households and during small social gatherings, according to Health Department contact tracing interviews, which is why the health commissioner recommended canceling holiday plans.

No matter when a vaccine is rolled out, Farley said, “We’re not going to get past this before Thanksgiving, Hanukkah or Christmas.”

Danya Henninger is director and editor of Billy Penn at WHYY, where she oversees the team, all editorial decisions, and all revenue generation — including the membership program. She is a former food...