Why getting the flu vaccine now matters even when protection is not perfect

Health officials say an annual flu jab can reduce serious illness, hospitalisation and death, with protection taking about two weeks to build.

As flu season begins, health officials are urging people who are eligible to get their annual shot without delay. Denise Hernandez, a public health professor at the University of Texas at Arlington, said vaccination is one of the simplest ways to protect not only individuals but also families and wider communities. The best window is now, she said, because the vaccine needs about two weeks to build full protection.

In the United States, flu activity usually runs from October through May and often peaks between December and February. That timing is one reason the Centres for Disease Control and Prevention recommends getting vaccinated by the end of October whenever possible. The agency says seasonal flu vaccines are matched to the strains researchers expect to circulate most widely in the coming months, even though those predictions can never be perfect.

The case for vaccination is not just theoretical. During the 2024-25 influenza season, the CDC estimated that flu shots prevented 10 million illnesses, 5 million medical visits, 180,000 hospitalisations and 12,000 deaths. The agency also says influenza still causes millions of illnesses, hundreds of thousands of hospitalisations and thousands, or sometimes tens of thousands, of deaths in an average season. Despite that toll, it says only about half of Americans receive an annual flu shot.

Hernandez stressed that the vaccine’s value is measured by more than its ability to block infection entirely. Because influenza viruses change from year to year, no seasonal shot can offer perfect protection, and effectiveness can vary depending on how closely the vaccine matches the viruses in circulation and on a person’s age and immune response. Even so, the CDC says vaccination can still reduce illness, hospitalisation and death, including in seasons when the match is less precise.

Safety remains a common concern, but public health officials say the flu vaccine has a strong safety record and is closely monitored. Most side effects are short-lived and mild, such as soreness where the injection was given, fatigue or a low-grade fever. The shot cannot give someone the flu, the CDC says, because it works by training the immune system to recognise the virus rather than causing infection itself. The CDC recommends annual flu vaccination for nearly everyone aged six months and older, with a few exceptions, including some people with particular medical conditions or a history of severe allergic reactions who should speak with a healthcare provider first.