Updated at 1:04 p.m. Nov. 2

Dr. Larry Rhein said he used to be able to set his clock to spikes in respiratory syncytial virus, known as RSV.

Cases used to spike in mid-October and all but disappear by April. But Rhein, the head of pediatrics at Worcester's UMass Memorial Children’s Medical Center, says that seasonality seems to be shifting — and more kids are getting severely ill.

RSV cases this year began to rise in mid-August, when 3% of RSV tests came back positive. Last week, the positivity rate had risen to 17%. And on Monday, about 85% of pediatric hospital beds in Massachusetts were full, according to the state Department of Public Health.

The virus usually causes mild, cold-like symptoms, but it can lead to hospitalization for infants and older adults. And public health measures taken to mitigate the COVID-19 pandemic also kept children from being exposed to other viruses, which doctors say is now causing more severe cases of RSV.

“What's challenging about this is that the symptoms of a common cold are common. And when that goes from mild to severe, it can be rapid,” Rhein said.

The surge in cases has parents, childcare providers and doctors in Massachusetts concerned. In Worcester, public health officials are advising people to wear masks indoors to prevent the spread of RSV, though the city has not mandated masking.

Parents like Kim Vanacore, who lives in Worcester, said she's a bit worried that her child could get seriously ill from RSV and not be able to get a hospital bed.

"I think we've all been a little frightened since COVID has been going on," she said. "And now you're hearing more about new symptoms of kids and hospitalizations."

RSV is usually transmitted through respiratory droplets or personal contact, although, unlike COVID-19, it can linger on hard surfaces for several hours, according to the Centers for Disease Control and Prevention. Parents or guardians of young children, especially those 6 weeks to 6 months old, should closely monitor cold symptoms. Experts say if a child develops a fever, decrease in appetite, or breathing problems on top of cold symptoms, that may be cause for concern.

Rhein is in favor of more people masking to prevent the spread of RSV.

“We're doing everything we can in the health care system, but we need the public's help to protect themselves and to protect us from being overrun with all these patients,” Rhein said. “So I do think the masking isn't too much of a price to ask for everyone's benefit, not just your own children and other people.”

The uptick in cases is concerning for day care providers, said Sharon MacDonald, executive director of the Guild of St. Agnes in Worcester County. Her organization cares for around 1,800 young children in day care centers and smaller home care sites.

“We have a number of our family child care homes that have three or four cases of it,” she said. “Similar to COVID, I think once it affects the children in the program, it does seem to be spreading. One of the concerns, of course, is there's a big stress on families when children are sick and a bigger stress on families when we ask children not to come in.”

The organization has asked staff and children to wear masks in the small vans and buses they use to get to and from their day care centers, but has not mandated masking, MacDonald said.

“I think everyone's tired,” she said. “We highly recommend flu vaccines and COVID vaccines and we are just really trying to keep the windows open and recommend masks, but not mandating yet.”

GBH News Worcester reporter Sam Turken contributed to this report.

Correction: This story was updated to correct the name of UMass Memorial Children’s Medical Center.