Why some people feel sick after the flu shot and what COVID boosters really mean for side effects

DCM Editorial Summary: This story has been independently rewritten and summarised for DCM readers to highlight key developments relevant to the region. Original reporting by The Conversation, click this post to read the original article.

Flu season has arrived earlier than usual this year, and health experts expect it to be more severe. Regardless of whether you’ve already received a flu vaccine or are considering getting one, there are a few important things to know. If you live in the UK and are at higher risk—such as being over 65, having a weakened immune system, working in health or social care, or being in contact with vulnerable individuals—you can get the vaccine for free. If you don’t qualify for a free jab, many pharmacies offer it for a fee. You should avoid the vaccine only if you’ve had a life-threatening allergic reaction to its ingredients, including egg-based components. In this case, make sure to inform your healthcare provider so they can offer you a suitable alternative.

It takes about two weeks for the flu vaccine to become fully effective, as your immune system needs time to produce protective antibodies. If you catch the flu shortly after getting vaccinated, it’s likely that you were already infected before receiving the shot – this is just unfortunate timing. If you have a fever or feel ill, it’s best to delay your vaccination until you recover. The vaccine doesn’t contain live virus, so it can’t give you the flu. You might feel mildly unwell or sore for a day after the jab, which is a normal immune response, not a sign of infection.

You can safely receive the COVID booster at the same time as your flu shot. Both vaccines remain effective, and side effects are generally minor and short-lived. Ideally, you should get your flu vaccine in October or November, so it’s at peak protection when flu cases are most common. The vaccine typically protects you for about three months, so getting it too early may result in reduced immunity later in the season.

Even if you’ve already had the flu this winter, getting the vaccine is still a good idea, since it protects against multiple strains. For people with weakened immune systems, the flu vaccine may offer lower protection, but it’s still beneficial. Older adults will usually be offered a high-dose version to help boost their immune response. It’s also a good idea to encourage those close to you to get vaccinated, which adds a layer of protection for you through reduced exposure.

Flu viruses are constantly changing, but scientists around the world, coordinated by the World Health Organization, work year-round to predict the strains most likely to spread in each region. They track flu activity in the southern hemisphere to anticipate what might hit the UK during winter. This well-established system generally works, though some years the vaccine matches the circulating virus better than others. Even if it’s not a perfect match, getting vaccinated can still reduce your risk of severe illness and help protect those around you.