The spread of H1N1 flu is slowing down in Korea after a peak in the first week of November, according to figures released by the Korea Centers for Disease Control and Prevention on Wednesday. Assuming the trend continues, H1N1 infections are projected to decline sharply in mid-December, when the vaccination in schools is to be completed.

One indicator is the number of prescriptions for oseltamivir, which is believed to help cure the infection. In the first week of November, prescriptions averaged 100,000 a day. In mid-October, daily prescription rapidly jumped from 3,400 cases to 67,000, and then dropped to 87,000 cases to 48,000 in the early to mid-November period. The number of schools closed for H1N1 flu shrank from 488 on Nov. 5 to 17 on Nov. 24.

Various factors contributed. "An infectious disease is bound to peak and then slow down after a certain period because the more people are infected, the more people develop immunity, and that was the situation in early November," said Kim Woo-joo, professor of internal medicine at Korea University Guro Hospital. But Kim cautioned that there are still more H1N1 patients than in mid-October and urged people to stay alert until the vaccination is completed.

Variants remain. To begin with, viruses resistant to drugs may emerge. Over 50 resistant cases have so far been reported worldwide. Resistant viruses through man-to-man infection have recently been reported in the United States and Britain. The more oseltamivir is used, the greater the chance for viruses to develop resistance. And here the drug has been prescribed for some 3 million patients so far.

There are also fears of a mutation. Two mutated viruses reaching deeper in the respiratory system, have recently been reported in Norway and Ukraine. The World Health Organization claims that the mutated viruses cause no serious problem because they can be prevented by the vaccines now in use. But it is watching out for others.