Influenza continues to rage across Japan, with an estimated 1.67 million people becoming sick and seeing doctors in the week through last Sunday, double the level the week before, according to health ministry data released Friday.

Authorities are calling for utmost vigilance as the nation heads into the New Year¡¯s holidays, when many people plan getaways, family reunions and other gatherings.

All 47 prefectures reported an increase in the number of patients, with the average number of flu cases per clinic per week surging to 42.66, more than double the level recorded the previous week.

Health authorities issue flu warnings when case numbers top 30 per institution per week. In the latest data, cases in 44 prefectures are now above that level.

Infection levels were especially high in Kyushu, with Oita Prefecture hitting the highest level of 82.64, followed by Kagoshima Prefecture (65.57) and Saga Prefecture (61.62). Chiba, Fukuoka, Aichi and Yamanashi prefectures followed, at 60.03, 59.86, 56.79 and 55.63, respectively.

Flu viruses are spreading across generations, but particularly quickly among children and teens, with nearly 800,000 of the new patients being those under 20.

Oita officials said the number of cases in the prefecture is the highest since 1999, when it started taking such statistics.

Masaki Inoue, executive director of the Oita Medical Association, a group of private practitioners in the prefecture, attributed the explosive growth in infections to weakened immunity against flu viruses among people.

¡°For the last few years many people prioritized COVID-19 vaccines over flu vaccines,¡± he was quoted as telling reporters on Wednesday. ¡°This may have resulted in a lowering of immunity for influenza.¡±

Many hospitals and clinics will close during the New Year¡¯s period, but local medical associations have doctors in private practice who will take turns accepting patients. Their schedules are listed on the websites of municipal governments.

Officials urge people to practice basic hygiene measures, such as frequent hand-washing and sanitizing, masking where it¡¯s effective and proper cough etiquette. They also recommend maintaining a room temperature of 20 to 25 degrees Celsius and humidity levels of 50% to 60%.

High-risk people, such as the elderly and those with underlying conditions, can benefit from getting vaccinated, which has been shown to prevent symptoms from getting worse. Fees for the shot vary but are generally between ?3,000 ($19) and ?5,000. Many municipalities offer subsidies of between ?1,000 to ?2,000 per shot.