The number of COVID-19 cases among children and teenagers in the state has risen sharply, with 10- to 19-year-olds seeing a nearly 60 percent jump in a report last week from the Massachusetts Department of Public Health.
At the same time, levels of coronavirus detected in Eastern Massachusetts waste water also have shot up in recent days, suggesting infections are on the rise in the region.
Experts said the latest COVID figures among youths likely reflected the effect of students going back to school and spreading the virus to each other.
The start of school leads to “a lot more social mixing. The last couple of falls we’ve definitely seen a big spike when schools started up. Some of that is certainly what we’re seeing now,” said Andrew Lover, an assistant professor of epidemiology in the School of Public Health and Health Sciences at the University of Massachusetts Amherst.
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Though the percentage increase is high, the case numbers are relatively low. The number of confirmed cases over a two-week period among 10- to 19-year-olds increased from 1,191 to 1,901, the highest number since early June.
Children ages 0 to 9 saw a smaller increase in cases: 15.1 percent. Within that number, though, there were two different stories: an 8.7 percent increase among children ages 0 to 4 and a 27.8 percent increase among children 5 to 9.
Lover said children 0 to 4 likely saw a smaller increase than those slightly older because the youngest children are staying at home or are in year-round day care.
The two-week case counts released by DPH ran up to Sept. 17. The numbers will be updated by another week when the state releases new data on Thursday.
Matthew Fox, a professor of epidemiology and global health at the Boston University School of Public Health, said in an e-mail, “I do think it is not surprising that with summer over and school being back in session that there are rising cases. I do think that school is the most likely explanation.”
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The DPH releases a two-week count of confirmed cases, broken down by age, every week. The total for all age groups increased by 12.6 percent. Other age groups saw increases of less than 8 percent, with the exception of 20- to 29-year-olds, who saw a 16.9 percent increase.
Experts have cautioned there may be a COVID-19 rebound during the fall and winter and have urged people to get vaccinated to protect themselves and others, as well as take other precautions.
Experts say children generally develop less severe disease than older adults, but some do get serious cases, and the possible long-term effects of cases are not fully understood. Children can also transmit the disease to others at school or at home. Vaccinations are recommended for children 6 months and older.
Fox said it was likely that the increase in COVID-19 cases among youth would “mean more cases in the elderly, but we don’t know that it will be a lot, and we know that that [elderly] population is the most likely to be vaccinated and boosted.”
Lover said he expected the case increases to “quiet down in the next month or so, but then probably when people start to travel a lot for the Thanksgiving and December holidays, we’ll see some upticks.”
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“It’s worrisome,” he said. “We could certainly be in for some surprises.” The big question, he said, is whether a new variant will emerge that can evade the immunity the population has developed from vaccinations and prior infections.
The state’s official case numbers do not include the results of widespread at-home rapid testing. The state has also discontinued its program of testing and reporting COVID-19 cases in schools.
“I would have preferred we still had counting of cases in schools just because they serve as a sign that an increase in cases is coming (as does the wastewater data) but at the same time, cases is probably no longer the best metric and we need to see what is happening with severe cases. I’m watching the hospitalization data more closely,” Fox said in an e-mail.
Lover also said he was keeping an eye on hospitalizations, which are currently on a plateau and have not shown signs of taking off.
Massachusetts Teachers Association President Max Page said in a statement that the “rising case counts are not surprising, since educators anticipated this development with the return of students to our public schools and colleges.”
“We have been consistently advocating for the state to remain engaged in supporting communities, schools and colleges with access to testing and vaccines,” Page said. He also said that districts and campuses that feel it is necessary to mandate mask use “should be supported in that decision.”
Dr. Mary Beth Miotto, president of the Massachusetts Chapter of the American Academy of Pediatrics, said, “I don’t think any of us are surprised” by the case increases. “I think we all knew we would run into this in the winter and even in the fall.”
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Respiratory viruses make a comeback during colder weather, said Miotto, who practices at the Mattapan Community Health Center. “The temperature starts going down. People go inside and they spread it,” she said.
The COVID-19 increases come as other childhood respiratory illnesses are already “skyrocketing,” she said.
Miotto urged people to take advantage of tools that science has provided that weren’t available earlier in the pandemic, such as getting their children vaccinated and boosted, and getting them tested if they develop symptoms. “We need to use the tools that we have that can make us feel — and truly be — safer,” she said.
She urged parents to seek out answers if they have any questions about getting their children vaccinated, saying information is available from doctors, nurses, school nurses, local public health officials, pharmacists, and federal and state public health websites.
The latest waste water data posted by the MWRA covered samples taken up until Monday. The numbers had been fluctuating for several months, but began rising on Sept. 21.
Andrew Lover, an assistant professor of epidemiology in the School of Public Health and Health Sciences at the University of Massachusetts Amherst, said the uptick was concerning, but “we’ll need another few sampling days to see if the trend continues before making any firm conclustions.”
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Waste water from 43 communities, including Boston, converges at the MWRA’s Deer Island plant for treatment. The sewage is tested for traces of the deadly virus. The MWRA reports numbers for both the southern and northern sections of its system. The testing determines the number of SARS-CoV-2 RNA copies per milliliter of waste water.
The northern MWRA section saw five days of increases. The seven-day average count reached 1,016 copies/mL of the virus on Monday. The number had been as high as 1,273 on May 17 during a spring bump in infections. But it had been as low as around 100 in March during a lull after the winter’s devastating Omicron surge.
In the southern section, the seven-day average count rose over the same five-day period, ending at 993 copies/mL on Monday. The number had gone as high as 1,332 on May 17. In March, it had dropped into the low 90s.
Martin Finucane can be reached at martin.finucane@globe.com.