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When getting the influenza vaccine, the most common concern is the "side effects," particularly pain at the injection site and fever. Knowing the symptoms and duration of side effects beforehand can help you approach vaccination with peace of mind.Based on actual 2025 data, nearly 80% of recipients experience "injection site pain," but this usually subsides within 1 to 3 days. Systemic side effects (malaise, mild fever) occur in about 10% to 25% of people, but the duration is short (1 to 2 days) and is far milder than the high fever from actual influenza (39–40°C lasting 5 days).

When people think of the influenza vaccine, most picture the "domestic" standard vaccine. However, as of 2025, high-efficacy imported vaccines like the "High-Dose Fluad®" and the "Recombinant Vaccine" are rapidly becoming common in Japan.The biggest difference between domestic and imported vaccines is the amount of antigen and the presence of an adjuvant (immune-enhancing agent). Imported vaccines, especially Fluad®, enhance these components, creating a significant difference in preventative effect compared to standard domestic vaccines, particularly in preventing hospitalization among the elderly.

For seniors aged 65 and over, the influenza vaccine is the most important tool for protecting their lives.As of 2025, the most highly recommended option for this group is "Fluad®," which contains four times the antigen volume of a standard vaccine and includes the immune-boosting agent "adjuvant (MF59)." Large-scale international studies have confirmed that Fluad® has overwhelming effectiveness compared to the standard vaccine, with an illness prevention rate that is 1.5 to 2 times higher and a hospitalization prevention rate of approximately 70% to 90%.Crucially, while Fluad® is expensive, most local municipalities offer subsidies, meaning the actual out-of-pocket cost is often comparable to that of a standard vaccine.

Every year when you go for your flu shot, you might wonder, "Is the standard vaccine good enough?"Influenza vaccines in Japan are evolving. Specifically, the "High-Dose Vaccine (Fluad®)" exclusively for those aged 65 and over has four times the antigen volume and significantly better effectiveness. Furthermore, the "Recombinant Vaccine" is debuting in Japan for the first time for adults aged 18 and older, greatly expanding the options for vaccination.

"If I get it every year and still catch the flu, is it pointless?" "Do healthy people need the shot?"Deciding whether or not to get the influenza vaccine is a perennial question. However, based on 2025 data, the vaccine's greatest value is clearly in "preventing severe illness" rather than just "preventing infection."For the elderly, pregnant individuals, and those with underlying conditions, it is an essential preventative measure—an "absolute must-do"—that significantly reduces the risk of hospitalization and death. Even for healthy adults and children, it offers major benefits in preventing absences from work or school closures, and stopping the spread of infection to family members.

Children under the age of 13 require two doses of the influenza vaccine to build solid immunity. Receiving these two doses at the correct interval is the key to achieving maximum preventative effect.The latest recommendation in Japan as of 2025 is for an interval of "2 to 4 weeks." Specifically, what doctors honestly recommend as the timing that yields the "highest antibody level" is exactly 4 weeks later. Conversely, if the interval is shorter than the minimum of "at least 2 weeks (13 days)," the vaccination is not covered by insurance and the effectiveness is drastically reduced.

"I got the shot, but when will it start working?" "What happens if I catch the flu right after getting vaccinated?"The influenza vaccine does not take effect immediately after administration. It requires a certain period for antibodies to be created in the body, enabling it to fight off the influenza virus. Knowing this "time until effectiveness" is a crucial point for protecting yourself in time for the peak of the epidemic.

"When is the most effective time to get the influenza vaccination?"It is generally said that antibodies begin to form about two weeks after the influenza vaccination, and the protective effect lasts for about five months. Therefore, it is crucial to determine the timing so that the antibody level in your body reaches its peak just as the influenza season begins in Japan, typically from late December to March.

When you develop a fever and visit a hospital, the influenza testing performed will primarily involve two types: the "Rapid Antigen Test" and the "PCR Test (Genetic Test)."The Rapid Antigen Test is appealing for its convenience, offering results in 15 minutes, but it carries the risk of false negatives. This accuracy can plummet to below 50% after 48 hours post-onset. In contrast, the PCR test boasts nearly perfect accuracy, ranging from 99.5% to 100%, but it comes with the drawbacks of time and cost.

When you go to the hospital with a fever, the test you will almost certainly receive is the "Rapid Antigen Test (the 15-Minute Test)." This inexpensive and fast-acting test remains at the forefront of influenza diagnosis as of 2025.However, behind its convenience lies a limitation: the possibility of a "false negative" (being negative when you actually have the virus). Its accuracy significantly decreases, especially for Type B influenza and when the test is performed more than 48 hours after the onset of symptoms.