Swine flu vaccinations began Monday with doctors and other medical professionals given priority because supplies are limited.
About 1 million medical workers are to be given the domestically produced vaccine first, according to the health ministry's priority ranking.
Next will be pregnant women and people with specific chronic diseases, beginning in November, in line with the government's plan to combat the fast-spreading H1N1 flu virus.
Children between the ages of 1 and 8 will be vaccinated starting in December, and parents of infants early next year.
But the Health, Labor and Welfare Ministry could accelerate the schedule later this week, as medical experts agree a single shot of the vaccine, instead of two as previously thought necessary, is considered enough for people aged 13 and older.
Among medical workers who were administered the vaccine Monday morning, Kazuko Naganuma, a 61-year-old nurse who got a 0.5-milliliter dose, welcomed the prioritizing. "It is good because we come into contact with patients," she said.
Naganuma was one of 15 staff members to be inoculated at a medical and pediatric clinic in Kofu, Yamanashi Prefecture, which received five 1-milliliter bottles of the vaccine Friday.
The clinic's director, Toshio Inoue, 70, said about possible side effects, "There are worries about giving a shot to people with underlying conditions, but I think any critical moment can be overcome."
In vaccinating the professionals who treat patients of the highly contagious disease, the ministry plans to survey about 20,000 of them for any side effects.
Sixteen prefectures have said they couldn't start inoculating health care workers Monday as scheduled due to delays in preparation, but most should be ready sometime this week.
The 16 prefectures were Hokkaido, Fukushima, Tokyo, Gunma, Kanagawa, Saitama, Aichi, Toyama, Shiga, Nara, Kochi, Fukuoka, Saga, Nagasaki, Kumamoto and Miyazaki.
