Flu prevention for residents of Hanoi who survive the winter using the world standard method of vaccination

As of October 2025, the incidence of influenza infections, especially among children, is increasing in Hanoi, Vietnam. In some schools in the urban area, a decision to suspend classes is also being considered. In Hanoi, due to the tropical climate, there are two waves of flu each summer and winter, and prevention awareness throughout the year is required more than in Japan.
Especially at this time of year, when the risk of infection increases, is an important time. The World Health Organization (WHO) and the Vietnamese Ministry of Health recommend annual vaccination for everyone over six months of age, and residents of the country are no exception. Vaccination reduces the risk of developing the disease by approximately 60%, and even if you do get sick, it can prevent complications such as pneumonia. Side effects are minor, and most of them are only temporary pain or mild fever at the injection site.
Now, what’s not quite as well known is the difference in vaccination methods between Japan and other countries. In Japan, subcutaneous injections are common, but in many countries abroad, including Hanoi, intramuscular injections are the standard method. This difference actually has a significant impact on how side effects manifest themselves. The subcutaneous tissue contains many immune cells, making it more susceptible to a strong reaction to antigens. Therefore, subcutaneous injections tend to cause redness, swelling, itching, and a feeling of heat. The Japanese Ministry of Health, Labor and Welfare also recognizes that subcutaneous injections have a “strong local reaction.”
On the other hand, muscle injections are administered to muscle tissues with abundant blood flow, so the antigen spreads quickly and local inflammation is suppressed. In fact, in a Japanese study, it was reported that muscle injections caused about half the pain compared to subcutaneous injections, and the frequency of swelling and redness was also significantly lower. Additionally, the incidence of influenza after vaccination was reported to be approximately 27% lower in the muscle injection group, and the protective effect was reported to be equivalent or even higher.
Personally, I have already had a vaccination with a muscle injection this year; whereas with the previous subcutaneous injection, I experienced redness, swelling, and a fever, this time I only experienced mild muscle pain for about 1 to 2 days. In the coming seasons, let’s get ready for the winter peak of infection with a world-standard vaccination with confidence.

In Hanoi, autumn and winter are the peak months for flu outbreaks. The best time to get vaccinated is between October and December.

In other countries, muscle injections are the mainstream method, and it has been proven that the side effects (swelling and tenderness) are milder than subcutaneous injections.

Subcutaneous injections are administered to areas of the skin with a high concentration of immune cells, making them more likely to produce a reaction; muscle injections, however, are safe and effective.

Japan Green Vietnam Clinic
Japan Green Vietnam Clinic

Dr. Kobayashi Hyo-no

He holds the Japanese Board of Medical Specialists certification in Respiratory Medicine, Allergy, Infectious Diseases, and Internal Medicine. After graduating from Hokkaido University, he trained at Tono-mon Hospital and worked as an assistant professor at Keio University before obtaining a medical degree. After working at the National Cancer Center in Singapore, he obtained a Vietnamese medical license and is currently working there.