Flu is the bigger concern. Infectious-disease experts say that an early season for influenza does not necessarily mean a long or severe one. They also say the predominant flu strain so far this year is the same as in 2003 - the earliest, and ultimately one of the worst, flu seasons in the past 15 years.
"Just tell people that [they] need to get vaccinated now. They don't have time to wait this year," said Lyn Finelli, chief of influenza surveillance and outbreak response at the Centers for Disease Control and Prevention.
There is no vaccine for RSV. Respiratory syncytial ( sin-SISH-uhl) virus is the most common cause of bronchiolitis, an inflammation of the small airways in the lungs. It can cause serious illness in young children and the elderly; about 100,000 children under a year old are hospitalized annually in the United States.
Victoria Brogan got a call from day care two weeks ago to pick up her 15-month-old daughter, Lyla, who had a low fever. Within three days she was coughing so hard she was vomiting.
The emergency room gave her breathing treatments - there are no antivirals for RSV - and sent her home to Moorestown with albuterol and steroids. After watching the girl's belly, not just her chest, move up and down as she struggled to take 69 breaths a minute, Brogan had her admitted to Virtua Voorhees last Wednesday.
"Knowing that she is not getting enough oxygen, it is frightening," said Brogan, whose daughter also had pneumonia. Lyla was released Sunday.
An abundance of respiratory viruses circulate from early fall through late spring. They often dance around each other in overlapping waves, one rising as another falls, each taking its turn dominating a community of human hosts.
There are exceptions to this winner-take-all pattern. In early fall 2009, the out-of-season pandemic flu arrived amid an unusually large outbreak of rhinovirus, the most common cause of colds. Neither backed down, although the flu caused the most severe illness.
The last early seasonal flu was in 2003. The H3N2 strain, which is known to cause more disease than other seasonal flus, did most of its damage by New Year's Day, ultimately causing an estimated 48,000 deaths nationwide - more than twice as many as the pandemic. RSV did not top out until February.
RSV's longer season usually overlaps with the flu, but "they rarely peak at exactly the same time," said epidemiologist Cecile Viboud, who studies viral transmission at the National Institutes of Health.
The highest incidence of flu is among school-age children. The second-highest is among children under 5 - also the most likely targets for RSV. "So a pediatric emergency room is going to be overrun if RSV and flu [peak] at the same time," the CDC's Finelli said.
So far this season, the flu - again early and H3N2 - is rising in tandem with RSV in Philadelphia. Based on past patterns, neither is near its peak. Will they continue on the same incline, or will they split up?
Even with flu barely starting in New Jersey, the recently expanded Cooper University Hospital emergency room in Camden recorded the most patients in its history on Monday, a spokeswoman said, driven partly by a rise in pediatric patients. Mondays typically are the busiest ER days; the previous Monday was No. 2.
St. Christopher's Hospital for Children has been experiencing some of its busiest days since the 2009 pandemic, much of it due to underlying asthma worsened by viral illness, said emergency department director Christopher Haines. The ER on Monday saw 289 patients, 55 percent more than the same day last year.
Excluding the pandemic, three of the five busiest days in five years at Children's Hospital of Philadelphia's emergency room have been the last three Mondays. No other December day in five years made the top 20.
Caroline Johnson, director of disease control for the Philadelphia Department of Public Health, has a future concern: "Early flu seasons have the potential for being bad flu seasons because fewer people may have been vaccinated when the disease starts to circulate."
Still, the situation is very different from 2003. That year, the vaccine was a poor match with the circulating flu viruses. Vaccine strains must be chosen months ahead based on predictions of which will emerge, and this year the scientists appear to have gotten it right.
More significant is that far fewer people were immunized in 2003; the government did not even track rates for children. This year, an estimated 37 percent of all Americans were vaccinated by November, including 50 percent of children under 5. More than 126 million doses have been distributed.
Respiratory viruses generally are spread through sneezing and touching. "People should take precautions: hand washing, covering their mouths," said Susan Gerber, leader of the CDC team that follows non-flu viruses, most of them without vaccines.
Several doctors here said such viruses accounted for most of what they had seen so far, with flu just starting to pick up.
Sudden weather changes and other factors over which parents have no control make a child more vulnerable, so pediatricians work with what they can control.
"Being exposed to secondhand smoke, thirdhand smoke [clinging to furniture], predisposes them to more severe respiratory illnesses and even more ear infections," said Denise Salerno, a member of a large Temple University practice.
The day before, she said, a mother brought in her 13-month-old baby for follow-up after a hospital stay for bronchiolitis caused by RSV.
"I wrote a nicotine patch for her," Salerno said.
Pediatric ER's Busiest Days
The last three Mondays were among the five busiest days in five years at Children's Hospital of Philadelphia's ER, excluding the fall 2009 flu pandemic.*
Rank, date, number of patient visits:
1 Dec. 17, 2012 376
2 March 9, 2009 373
3 Dec. 10, 2012 370
4 Feb. 7, 2011 362
5 Dec. 3, 2012 359
* The pandemic has accounted for 12 of the top 20 days since 2007, including No. 1 (Oct. 26, 2009, with 570 visits) through No. 8 (Oct. 23 and 29, 2009, tied with 378 visits).
Flu vaccine is available at doctors' offices, retailers, and health departments. Go to www.philly.com/flu
Contact Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.