In the case of the COVID vaccine, side effects can be interpreted as a good sign. Side effects from the vaccine are due to your body mounting an immune response, and everyone’s immune response is different based on their overall health.

Younger people have been found to be more likely to develop side effects, possibly due to their more robust immune system.

That doesn’t mean you’ve got a weaker immune system or that your body isn’t responding well to the vaccine if you don’t get side effects, It just means that different people’s bodies can respond differently to the same stimuli.

Although side effects are in themselves nothing to worry about, it helps to be mindful of their type, intensity, and duration. For most people, any side effects they experience will be moderate. 

Figuring out what kinds of side effects someone might get and how long they may last isn’t an exact science. Everyone responds to vaccines differently, so it’s an exact science. Everyone responds to vaccines differently, so it’s hard to predict if someone will develop slide effects and what type. However, the majority of side effects will resolve within one or two days. 

Across clinical trials for the Pfizer, Johnson, and Johnson, and Moderna Covid Vaccines, scientists found similar trends. The most common side effects of the Johnson and Johnson vaccine are headaches and fatigue, which is also true of the Moderna vaccine side effects. The same occurred with Pfizer vaccine side effects, with all three vaccine types also commonly causing varying rates of injection site pain, fever, and muscle soreness. 

Local and Systemic side effects

In the study, researchers from the United Kingdom used data from a COVID-19 symptom app to look at side effects experienced by over 627,000 people who received either the Pfizer-BioNTech or AstraZeneca-Oxford COVID-19 vaccines.

Among people who were vaccinated, 25.4 percent or 1 in 4 people reported having a systemic side effect, one that occurs in a part of the body other than near where the vaccine is injected. The most common systemic side effects were fatigue and headache. They generally appeared within the first 24 hours after vaccination and lasted on average about 1 day.

Local side effects were much more commonly occurring in 66.2 percent of people with tenderness and pain near the injection site most frequently reported. These generally began the day after injection and lasted about 1 day. Other side effects such as rashes, a burning sensation on the skin, or red welts on the lips and face, were reported by a small number of people less than 2 percent.

First dose versus the Second dose

For the Pfizer-BioNTech vaccine, systemic side effects were worse after the second dose  22 percent of people reported side effects after the second dose, compared to 11.7 percent after the first.

Systemic side effects were more common after the first dose of the AstraZeneca-Oxford vaccine 33.7 percent. Researchers didn’t look at data on side effects after the second dose of this vaccine.

For people who received two doses of the Pfizer-BioNTech vaccine, local side effects were slightly less frequent after the second dose (68.5 percent) than the first dose (71.9 percent). Local side effects were also less common after the first dose of the AstraZeneca vaccine (58.7 percent).