Why it's a good thing if you feel unwell after your COVID-19 booster jab

What are the side effects of the COVID-19 vaccine - and why might they be a good sign?

Hospital admissions and deaths linked to COVID-19 are on the rise in the UK. Getting the COVID jab is the best way to protect yourself from becoming seriously unwell with the virus - and people eligible for their booster jabs are being urged to get them as soon as possible.

Guidance released in September 2023 advises that even if you have had COVID-19 recently, if you feel better, you don't have to wait 4 weeks to get your booster jab anymore.

Side effects from the jab are common, particularly in the first day or two after being vaccinated. These include soreness in the arm where you had your jab, feeling tired, headaches, aches and pains, nausea, and chills.

And while these symptoms aren’t pleasant, a new study - which is yet to be peer reviewed - suggests they are a sign that your body is responding properly to the jab.

Researchers at the University of California, San Francisco, studied more than 300 people to find out how their bodies responded to the COVID-19 vaccine¹. They found that those who experienced chills, a fever or headaches after the COVID jab created more antibodies against the virus compared to those who didn’t have any side effects. Those who had warmer skin and an increased heart rate after the jab were also found to have more antibodies.

Put simply - the more symptoms you have, the more antibodies you’ll make. And if you have more antibodies, your immune system will have a better chance of fighting the virus if you become infected.

What are antibodies?

An antibody is a protein produced by the body's immune system when it detects harmful substances, called antigens. Examples of antigens include microorganisms like viruses, bacteria, fungi and parasites, as well as chemicals. When antibodies detect and attach themselves to an antigen like a virus, it tells the other parts of the immune system to attack the invaders. This is how our bodies develop immunity to a disease.

William Schaffner, a professor of preventative medicine at Vanderbilt University, explains that booster vaccinations stimulate the immune system to raise antibody levels. "Higher antibody levels generally are associated with a longer duration of protection against serious disease as well as some enhanced protection against variants," he explains.

How to treat COVID-19 vaccine side effects

If you feel unwell after having your booster, you should rest and drink plenty of water to stay hydrated. Over-the-counter medicines, such as paracetamol or NSAIDs like ibuprofen, can be used to treat headaches or general aches.

Rare side effects

Very rarely, more serious side effects can occur. Cases of inflammation of the heart - called myocarditis or pericarditis - have been reported very rarely after the COVID-19 vaccine. Most of the people affected have felt better and recovered quickly following rest and simple treatments.

Further reading

1. Dutcher et al: The more symptoms the better? Covid-19 vaccine side effects and long-term neutralising antibody response.