Cases spike among children in Mass.

Mass General Brigham for Children is seeing a similar pattern. Dr. Ari Cohen, chief of pediatric emergency medicine, said flu has become the dominant virus sending children and teenagers to the emergency department in the past two weeks.

“Everybody’s got the flu,” Cohen said. “That’s really what it seems like right now.”

Flu activity is picking up across the country, with several states, including New York and Rhode Island, experiencing “very high” rates of flu-like illness; Connecticut and Massachusetts are listed as “high,” according to the Centers for Disease Control and Prevention.

In Massachusetts, the share of all emergency room visits that resulted in a patient being hospitalized with flu stood at 1.9 percent as of Dec. 13, more than double the rate at this time last year. And outpatient visits to healthcare providers for flu-like illness reached 4.9 percent, up from 2.8 percent the previous week, according to the state’s weekly flu dashboard.

Levels of COVID-19 in Massachusetts and across the country remain low.

Dr. Larry Madoff, medical director of the Massachusetts Department of Public Health’s Bureau of Infectious Disease and Laboratory Sciences, said flu activity for the general population is rising as expected for this time of year.

“Every flu season is different, but this is typically when we start to see an increase in influenza activity,” Madoff said.

The surge in pediatric emergency visits comes as influenza A continues to dominate nationally. Public health officials are closely monitoring a newer flu strain known as subclade K, a subvariant of the H3N2 subtype of influenza A that has drawn attention for driving an earlier-than-usual spike in flu cases in the UK and several other countries.

Madoff said it is still too early to confirm how widespread subclade K is in Massachusetts because there is not yet enough sequencing data from state laboratories. However, most flu cases identified in Massachusetts so far this season are H3N2, and nationally, about 70 to 80 percent of sequenced H3N2 samples belong to subclade K.

“We fully expect that those will be part of that subclade,” Madoff said, adding that more detailed state data is expected soon.

Clinicians emphasized that specific subclades do not change how patients are treated in emergency settings. Hospitals typically test only for influenza A or B, not for subtypes or subclades.

“Clinically, it doesn’t change anything I do,” Cohen said.

Doctors say children arriving in emergency departments this season are often experiencing high fevers — frequently 103 degrees or higher — along with headaches, body aches, congestion, coughing, and fatigue. Teenagers and older adolescents often present with intense achiness and exhaustion, while younger children are more likely to struggle with respiratory symptoms, clinicians said.

Despite the sudden increase in emergency visits for influenza, clinicians say the surge appears to be driven by the number of children getting sick rather than a clear increase in severity. Dehydration and breathing difficulties remain the most concerning symptoms, particularly for infants and children with underlying health conditions.

Public health officials are also watching flu vaccination rates, which lag behind recent years. About one in three Massachusetts residents has received a flu shot this season, according to Madoff — lower than health officials would like to see.

Doctors say vaccination can make a significant difference even when it does not prevent infection entirely, reducing the likelihood that children will need emergency care or hospitalization.

“It’s not too late to get vaccinated,” Madoff said.

Madoff and Cohen say mixed messaging around vaccines may be contributing to lower uptake.

“When you have national bodies questioning the utility and efficacy of vaccines, people who are on the borderline are going to choose not to get them,” said Madoff.

Cohen said he is seeing the effects firsthand, with there being fewer vaccinated children in the emergency department.

“I think there’s probably vaccine fatigue,” said Cohen. “There are just so many vaccines out there, and especially with COVID, a lot of people feel like they’re over it. But we know the flu kills every year. We know the best protection out there is the flu vaccine.”

As cases rise, clinicians are urging families to focus on supportive care at home when appropriate — and to know when emergency care is truly needed.

McCoy also cautioned that emergency departments have been busy in recent days, with long wait times reported across the state. She urged families to seek emergency care for serious symptoms such as breathing difficulties or dehydration, but to consider urgent care or their pediatrician for milder illness when possible.

For children, she recommends frequent, small amounts of fluids to prevent dehydration, including electrolyte drinks, along with saline nasal spray and weight-based acetaminophen or ibuprofen to reduce fever and body aches. Honey can help soothe coughs in children over 1 year old.

By contrast, over-the-counter decongestants, combination cold formulas, and cough suppressants provide little benefit for young children and can cause side effects such as jitteriness, sleep problems, increased heart rate, or stomach upset, McCoy said. She said antibiotics do not treat viral infections like the flu and should only be used for confirmed bacterial infections.

Prescription antiviral medications such as Tamiflu may be helpful for certain patients — particularly young children and those with underlying health conditions. These medications do not cure the flu but can reduce the risk of complications and be especially beneficial for those with underlying health conditions. They should be taken in addition to the vaccination, according to McCoy and Madoff.

Families should seek emergency care if a child has difficulty breathing, signs of dehydration, confusion or lethargy, bluish lips or face, or persistent vomiting that prevents fluids. Children with milder symptoms who are drinking fluids and breathing comfortably can often be cared for at home or seen by a pediatrician or urgent care provider.

As flu season continues, doctors say the message is one of awareness rather than alarm: get vaccinated, stay home when sick, and seek care when warning signs appear.

Sadaf Tokhi can be reached at sadaf.tokhi@globe.com.