WHO alerts to flu variant K as cases surge across Northern Hemisphere

The World Health Organization issued an alert for the upcoming flu season, citing rapid growth in cases of influenza A H3N2 variant K since August across multiple countries. The variant, also identified as subclado K or J.2.4.1, represents a new genetic branch of the seasonal flu virus.

The timing of the outbreak coincides with the arrival of winter in the Northern Hemisphere, a period when flu cases and other respiratory infections typically increase, potentially straining health systems. The WHO noted that while infections remain within expected seasonal ranges in some regions, several countries have reported unusually early season starts and activity levels above normal thresholds.

In the United States, flu season arrived earlier than typical this year. New York City and surrounding areas, including Long Island and North Jersey, recorded some of the highest levels of flu-like illness in the country as of December 6, according to Centers for Disease Control and Prevention data. The city's acting health commissioner, Dr. Michelle Morse, said flu cases were "skyrocketing" in New York City and urged residents to get vaccinated and wear masks if symptomatic. Poly Prep Country Day School, a private school in Brooklyn, closed for two days after at least 200 students were absent or sent home with flu or flu-like symptoms.

In Spain, Catalonia reported sustained increases in flu incidence during the week of December 8 to 14, maintaining very high transmission levels. The region recorded 764 cases per 100,000 inhabitants, with the A(H3N2) variant K accounting for 80 percent of cases in week 48. The current incidence substantially exceeded peaks from the three previous seasons, with the upward trend beginning notably earlier than the prior year.

Madrid also reported rising cases, with a 14.5 percent increase in the week ending December 14 and a rate exceeding 308 cases per 100,000 inhabitants. The region recorded 22,110 cases that week, bringing the seasonal total to 57,670 cases.

In South America, Peru reported at least two imported cases of H3N2, and health specialists in Chile called for an epidemiological alert to strengthen surveillance at border crossings and health centers ahead of the possible arrival of the virus. Experts recommended reinforcing vaccination campaigns and monitoring at entry points.

Available data do not indicate that the variant K causes more severe disease than previous influenza strains. However, health authorities emphasized the importance of vaccination and continued epidemiological surveillance to monitor virus activity. Specialists noted that flu vaccine protection typically lasts six to seven months, meaning immunity may be waning, particularly among vulnerable groups including children, people with chronic illnesses, and older adults.

The WHO stated that some countries have reported early starts to the flu season, while in others influenza activity is beginning to increase but has not yet reached epidemic thresholds. Authorities called for countries to maintain vigilance and promote vaccination as the Northern Hemisphere enters peak flu season.