PHILADELPHIA — Last spring was the busiest season Michael K. Donohue can remember for his family’s six funeral homes in the Philadelphia suburbs. COVID-19 was the primary reason, of course, with 160 funerals in April alone — double the usual number.

But this fall, even as the daily totals of new infections have surged past where they were in the spring, business at Donohue Funeral Homes remains fairly normal — so far. Donohue, the president of the 122-year-old business based in Upper Darby, sees the trend as well as any epidemiologist.

“It’s just not hitting the elderly,” he said.

Since the start of the pandemic, younger people have been far more likely to survive infection, and for many weeks, they have accounted for the bulk of new cases, a Inquirer analysis finds. Other factors, such as improvements in medical care, are thought to have helped limit the recent death toll, but the demographic shift is clear — with 18-to-29-year-olds lately accounting for the largest share of new cases across all age groups.

Yet with colder, drier weather upon us and more people gathering indoors, infectious disease experts warn that the rise in infection is once again leading to an increase in deaths among the old and vulnerable. The size of this trend is still not clear, as people can die of COVID-19 weeks after the initial diagnosis, but concern has prompted local and federal health officials to urge the cancellation of large family get-togethers at Thanksgiving.

And while better precautions seem to have limited disease in nursing homes during the summer after an awful spring, case counts in these facilities are rising once again.

What’s more, despite the mounting number of illnesses nationwide, the vast majority of people of all ages remain susceptible to infection, a new analysis led by Pennsylvania State University scientists suggests.

The study’s authors estimated that through the end of October, fewer than 7% of Pennsylvanians had been infected with the coronavirus, including confirmed cases of illness and those who had no symptoms. In Rhode Island, the corresponding number was about 11%, and in Massachusetts, 8%, the Penn State researchers found, working with others at Brown, Yale, and Harvard Universities.

“That basically means that we are fully susceptible to a big winter wave,” said senior author Maciej F. Boni, a Penn State biologist who studies infectious disease.

For the Inquirer analysis, health officials in Philadelphia, Montgomery, and Gloucester Counties provided detailed, week-by-week data for each age group, showing that the share of new cases among people under age 30 has more than doubled since the spring. A similar pattern held true for Pennsylvania as a whole, according to data from the state Department of Health.

In Philadelphia from Oct. 1 through Nov. 12, people age 29 and younger accounted for more than a third of confirmed new cases — 4,858 out of 13,336. Yet from April 5 to May 5, they represented just 16% of total city cases.

People aged 70 and up, on the other hand, accounted for 19% of the city’s cases during those first weeks in the spring, and just 6% of cases in recent weeks. That trend was accompanied by a steep drop in city COVID-19 deaths among all ages, from 1,131 to 41.

In Montgomery County for roughly the same two time periods, those under age 30 represented 12% of cases in the spring and 35% in recent weeks. In Gloucester County, home to Rowan University, that younger age group accounted for just 14% of new cases from April 5 to May 2, when campus was mostly closed. From Oct. 16 to Nov. 12, when in-person classes had resumed, people under 29 accounted for 41% of cases.

COVID-19 deaths in all three counties remain below where they were in the spring. But Val Arkoosh, a physician who chairs the Montgomery County Board of Commissioners, warned that if younger people transmit the virus to their elders, the deadly trend of spring could recur.

“If case numbers continue to rise, we will continue to see an increase in hospitalizations, and eventually an increase in deaths,” she said at a briefing Wednesday.

A harder question to answer regards the death rate: that is, the likelihood that an infected person will die. The concept has prompted public confusion since the start of the pandemic, and not just because mortality varies so much across age groups and by whether people have underlying health conditions. Both the numerator and the denominator in that calculation can be difficult to determine for several reasons, allowing for politicians of various stripes to portray the crisis as better or worse than it may be.

One wrinkle is that epidemiologists refer to the deadliness of a virus in two different ways: the case fatality rate (what% of sick people die) and the lower infection fatality rate (what% die out of all who have been infected, including those with and without symptoms). The infection fatality ratio for those age 70 and up, for example, is estimated at 5.4%, but well below 1% for other age groups, according to the U.S. Centers for Disease Control and Prevention.

Another issue is how our ability to count cases has changed with time. Early in the pandemic, many infected people were unable to get tested, making for a smaller denominator. Yet some people who died of COVID-19 early on also did not get tested, impacting the numerator as well.

And when it comes to ascertaining the larger number of people who have been infected without symptoms, the quality of an estimate depends on how accurately researchers sample the population.

Exact answers may not come for a while, but rest assured, COVID-19 remains more deadly than the flu. And even if a person who is severely ill with COVID-19 does not die, the consequences can linger for months.

Many organizations have tracked the pandemic by number of hospitalizations, but that, too, can be prone to error. That’s partly because some state governments have poor systems for hospitals to report data, said Boni, the Penn State biologist, who has served on World Health Organization committees for the management of malaria and other disease epidemics.

What’s more, the decision of whether to admit a patient to the hospital is a judgment call; early on, some gravely ill COVID-19 patients were known to have stayed home when health-care systems were overwhelmed.

Another option is to measure what% of hospitalized patients become so sick that they must go to the intensive care unit, Boni said. By that measure, care seems to be improving. In Pennsylvania, hospitalized patients are now half as likely to progress to the intensive care unit, compared with the spring, his study found. Hospitals in Rhode Island and Massachusetts improved by that measure as well, but to a lesser degree, he said.

Still, COVID-19 treatments have not improved enough to fully explain the dramatic decline in deaths. Among the few drugs shown to reduce mortality are corticosteroids, and large health centers such as Temple University Hospital have been administering them to severely ill patients since March.

A primary reason for the decline in deaths remains simply the one Donohue sees at his funeral homes: the ages of those who are getting sick.

He and his employees worked round the clock in April and May to accommodate the families of elderly victims. The funeral homes were able to meet the excess demand, though they centralized all COVID-19 cases at the Upper Darby location to limit the spread of infection.

Things remain calmer, for now. But if a winter wave occurs, physicians warn that he will be busy again before long.

Tim Tai/The Philadelphia Inquirer/TNS Anthony Harvin wears a Santa hat as he receives a coronavirus test from Dante Terracciano of Philly Fighting COVID at Vine Memorial Baptist Church in West Philadelphia on Saturday. Staff from Philly Fighting COVID and the Children’s Hospital of Philadelphia offered free testing organized by the traveling Community-Accessible Testing & Education (CATE) program, which offers testing at different sites across Pennsylvania.