Schools across the United States and the United Kingdom are grappling with an early and aggressive flu season, as a fast-spreading variant of the H3N2 influenza virus—dubbed the "K" strain—drives up illness and absenteeism among students and staff. The surge has led to dramatic responses, including the temporary closure of St Martin's School in Caerphilly, Wales, where more than 250 pupils and staff members have fallen ill in recent days.
On December 6, 2025, St Martin's School announced it would shut its doors for a brief "firebreak" period after a "significant outbreak of flu-like illness among pupils and staff," as headteacher Lee Jarvis described in a letter to parents. The closure, designed to allow for a deep clean and reduce further transmission, will last until at least December 9, with online learning in place for all year groups during the interruption. The school reported 242 pupils and 12 staff members absent, with symptoms including vomiting, diarrhea, high temperatures, cough, headaches, fatigue, and general flu-like effects. "The average recovery period appears to be around seven days," Jarvis noted, emphasizing the school's commitment to health and safety. Public Health Wales confirmed it had been in communication with the school and supported the closure following a thorough risk assessment.
This local outbreak is reflective of a broader trend. According to The Hill, schools across the U.S. are also bracing for more illnesses and absences as the H3N2 "K" strain picks up speed. The variant has already led to increased hospitalizations in Canada and the U.K., prompting concerns that the 2025 flu shot may be less effective against it due to multiple mutations that differ from the vaccine’s target strain. Still, public health experts are urging people to get vaccinated, arguing that even a less effective shot significantly reduces the risk of severe illness, hospitalization, and death.
“Even an imperfect influenza vaccine—let’s say it shaves 30 percent off of your influenza risk—is still probably shaving even more off your risk of getting severe influenza, meaning you’re only 30 percent as likely to get the flu, but if you get the flu, your likelihood of getting severe flu is much, much lower,” Ryan Maves, professor of infectious disease at Wake Forest University, told The Hill. “Your likelihood of going to the ICU is much lower. Your likelihood of getting hospitalized is much lower, your likelihood of death is much lower. All of those things together cumulatively affect the risk and the efficacy of any given influenza vaccine and so, when we look at getting the flu shot this year, I don’t think it’s really a decision. The decision is, ‘How lucky do you feel?’”
Despite these strong recommendations, vaccination rates remain troublingly low. Unlike some required school vaccines, no U.S. state mandates flu shots for schoolchildren, and last year, fewer than half of students received a flu vaccine. This gap in coverage is especially concerning as the Centers for Disease Control and Prevention (CDC) recently reported a sharp uptick in flu cases: clinical labs documented a 5 percent case-positivity rate for all types of influenza in the week prior to December 3, 2025, up from 2.9 percent the week before. The rise is particularly noticeable among young people, a trend mirrored in the Caerphilly outbreak.
Schools and health providers in the U.S. report that they are receiving less federal support than in previous years, a shift some attribute to Health and Human Services Secretary Robert F. Kennedy Jr.’s public skepticism toward vaccines. Last flu season saw some of the highest hospitalization rates in recent memory, resulting in the deaths of more than 280 children, according to The Hill. The stakes are high, and the new "K" strain appears to be a particularly unpleasant version of the virus. "You'll usually develop [symptoms] about one to four days after being exposed, and it feels like being hit by a truck," ABC News chief medical correspondent Dr. Tara Narul recently said. "It's different from a cold, so you're going to have fever, muscle aches, headaches, chills, sore throat, cough, runny nose, stuffy nose, maybe even vomiting and diarrhea."
For school administrators, containing the spread of the flu is a daunting challenge. Experts recommend that students and staff stay home when sick, but this advice is hard to enforce amid widespread chronic absenteeism and ongoing staffing shortages. Lynn Nelson of the National Association of School Nurses explained to The Hill that schools once used a rise of more than 10 percent in chronic absenteeism as a warning sign of a flu outbreak. However, absenteeism has climbed so dramatically since the COVID-19 pandemic that this benchmark no longer provides a reliable indicator of flu activity. Instead, Nelson advises schools to focus on clusters of symptoms—such as sudden high fever—and to pay close attention to anecdotal reports from parents about why children are being kept home. "If you’re seeing clusters of cases, it could be influenza, then people need to pay attention to that," she said.
Back in Caerphilly, the decision to close St Martin’s School was not taken lightly. Headteacher Lee Jarvis acknowledged the disruption to families but stressed that "our priority is the health and wellbeing of all pupils, staff, and the wider community." The closure affects all year groups, but the school has moved quickly to implement online learning. Jarvis assured parents that the situation would be closely monitored and that updates would be provided as needed. Public Health Wales, which has been in contact with the school throughout the outbreak, confirmed that the temporary shutdown followed a careful risk assessment and was a prudent measure to limit further spread. Parents were urged to keep children at home if they showed any symptoms, "even if mild."
This swift response stands in contrast to the more limited tools available to many U.S. schools, where chronic absenteeism and reduced federal support have complicated efforts to identify and contain outbreaks. While the "firebreak" closure in Caerphilly is a dramatic step, it may offer a blueprint for other schools facing similar surges. Deep cleaning, temporary closures, and a willingness to shift to remote learning—even briefly—could help stem the tide of illness, especially as experts warn that the current flu vaccine may offer only partial protection against the H3N2 "K" strain.
As the world watches the progress of this flu season, the experiences of schools like St Martin’s serve as a stark reminder of the importance of vigilance, rapid response, and clear communication. With the "K" strain spreading quickly and vaccination rates lagging, public health officials on both sides of the Atlantic are urging communities to take every possible precaution—because when it comes to influenza, a little luck and a lot of preparation can make all the difference.
