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"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.

In the diagnosis of influenza, the test that boasts the highest accuracy is the PCR test (Real-Time RT-PCR). While the common rapid antigen test determines if a virus is currently in the body, the PCR test directly detects the virus's genetic material, making its accuracy over 99.9%.As of 2025, the PCR test is utilized as the "ultimate weapon" for cases where the result from a conventional rapid test is unclear, or for patients with a high risk of severe illness (such as the elderly and those with underlying health conditions).

When influenza is suspected, medical institutions primarily use the "Rapid Antigen Test," but there are also more highly accurate methods like the "PCR Test" and the newer "Immunochromatography Method." The type of test used significantly affects the time required to get results and, most importantly, the "Accuracy (Sensitivity)."

The most vexing issue with the rapid influenza antigen test is the "false negative" (being negative when you actually have the flu). A false negative not only delays appropriate treatment but also complicates the decision to return to school or work.Why do false negatives occur? It's due to a variety of factors, including the time elapsed since the onset of the fever, the virus type (especially Type B), and even the quality of the specimen collection.

Do you know how much a doctor actually relies on the result of a rapid test kit when diagnosing the flu? Many of the rapid antigen test kits used during the 2024-2025 season boast a sensitivity close to 90% for Type A influenza, but their sensitivity for Type B influenza tends to be lower, around 60% to 80%. This difference becomes a major issue, especially during the peak season.

When you have a high fever, the immediate urge is to rush to the hospital, but the accuracy (positive rate) of the influenza test is heavily influenced by the time elapsed since the onset of the fever (the start of the fever). If you go less than 12 hours after the fever starts, you are often sent home, as the positive rate for the rapid antigen test is very low. As of 2025, the iron rule for getting a confirmed diagnosis is to aim for the "12 to 36 hours after onset," which provides the most accurate results.

If you suspect you have the flu, rushing to the hospital immediately is often not the best move! The "12 to 48 hours (especially 12 to 24 hours) after the onset of fever" is the Golden Time for receiving an examination and test. This is when the rapid influenza test (rapid antigen test) achieves its highest accuracy, and when anti-influenza medications can exert their maximum effect.We will explain why going immediately after the fever starts can lead to a "false negative" result, and provide the latest guidelines for the 2024-2025 season, including what to do if more than 48 hours have passed. The general rule of thumb is: "Wait half a day to a full day after the fever starts, and go to the hospital before a full 24 hours has passed."