Points to consider for vaccination if you are considering a temporary return to Japan

Many people might think that the flu vaccine is only given in the fall. In Japan, December to March is usually considered the flu season, and it is recommended that people get vaccinated before the flu outbreak intensifies. However, in Hanoi, flu does not always only occur during the winter. Past surveys have reported that influenza is present throughout the year, and there is also a possibility that it peaks in summer and winter. Therefore, instead of thinking of “getting vaccinated every year in October,” it is important to consider the local flu situation, lifestyle, and travel plans when deciding when to get vaccinated.

Influenza vaccines come in “northern hemisphere” and “southern hemisphere” versions. The WHO recommends two vaccine strains for the northern hemisphere each year, in February and September, based on the epidemic situation in various parts of the world. However, in tropical and subtropical regions, it is recommended that people choose not only based on the hemisphere where they live, but also by considering the epidemic situation in the region and the virus’s compatibility with the prevailing strain at that time.

Currently (as of the end of August 2026), the "Influenza 2026" available in Hanoi is a vaccine that adopts the virus strain recommended by the WHO as "for the Southern Hemisphere in 2026". Meanwhile, the WHO has also announced a new composition for the Southern Hemisphere in February 2026. Since some of the recommended strains are different for the Southern Hemisphere and the Northern Hemisphere, it is important to consider which one to choose based on the perspective of "when and where you are most likely to be infected."

So, when should Hanoi residents get vaccinated? If you spend most of the year in Hanoi, you don’t need to limit yourself to the Japanese winter. Since infection is possible year-round in Hanoi, it’s essential to consider the vaccine available at that time and the local epidemic situation rather than simply waiting until the Northern Hemisphere version is available.

Meanwhile, with plans to return to Japan temporarily from December to February during the winter break and Tet holidays, if the 2026–2027 Northern Hemisphere vaccine is available in Hanoi to prepare for the winter season in Japan, it would be an option. Since it takes approximately two weeks to develop sufficient immunity after vaccination, if you plan to return home, it would be a good idea to consider getting vaccinated at least two weeks before departure.

Even if everyone returns to Japan in the winter after receiving the vaccine for the southern hemisphere in August or September, it is not necessary for everyone to receive an additional vaccination for the northern hemisphere. Australia’s vaccination guidelines state that, considering factors such as length of stay, risk of severe illness, and the difference between the vaccine strains for the southern and northern hemispheres, additional vaccination can be considered. For example, for healthy adults who received the southern hemisphere vaccine in August and returned home for a short period in December, and elderly people who return home for a long period in February the following year or those with underlying medical conditions, the reason for considering additional vaccination may be different.
It is also important to note that vaccination rates in Japan and overseas differ. In Japan, children under 13 years old are generally required to receive two doses; however, the U.S. CDC and others require children aged 6 months to 8 years to receive one or two doses depending on their previous vaccination history; for children aged 9 years and older, the standard is one dose. In the case of your child, it is also important to consider not only their age but also their previous influenza vaccine history when determining the number of doses they should receive.

Flu vaccines are important to consider not only when to get vaccinated, but also where to live, where to travel, and which composition of vaccine to get. Instead of simply following the same schedule as Japan, make your vaccination plan in consultation with a healthcare provider based on your own and your family’s lifestyle and travel plans.

Flu is common throughout the year in Hanoi, so we consider the timing of vaccination based on the current situation and our daily lives and travel plans.

For those returning to Japan in February and March, Northern Hemisphere-use vaccines are also an option. For those who have already received the Southern Hemisphere vaccine, additional vaccinations will be determined individually based on factors such as the time since vaccination, length of stay, age, and underlying medical conditions.

Children should be careful about the number of doses they receive. In other countries, the standard is generally that children between 6 months and 8 years old receive one or two doses depending on their previous vaccination history, and that children aged 9 years and older receive one dose.

Japan Green Vietnam Clinic
Japan Green Vietnam Clinic

Dr. Hayoichi Kobayashi, Internal Medicine

Certified General Internal Medicine Specialist, Respiratory Specialist, Infectious Disease Specialist, and Allergist by the Japan Medical Specialists Organization. Doctor of Medicine. Graduated from Hokkaido University School of Medicine. Received a Ph.D. in Medicine from Keio University. After completing internal medicine training at Tono-mon Hospital, he has been engaged in comprehensive internal medicine, respiratory medicine, infectious disease, and allergy care for over 10 years, primarily at Keio University Hospital. Subsequently, he has been involved in clinical and research work at the National Cancer Centre Singapore. Starting July 2025, he is providing medical care for Japanese people in Hanoi.