The US’ Food and Drug Administration’s (FDA) data shows that age, manufacturer, and dose are main factors in determining side effects in Covid-19 vaccines.

Mild to moderate side effects are extremely likely after receiving any of the two vaccines being administered in Qatar, according to statistics by the FDA.

Side effects in Pfizer-BioNTech and Moderna vaccines are often a sign that the body is building immunity to the virus, meaning the shot is working as it should. 

These typically show up within 12 to 24 hours of getting the vaccination and do not last more than a few days.

“It’s important to remember: When people get side effects from vaccines, it’s not really because of the vaccine; it’s more of the body’s immune response to the vaccine,” Dr. Vivek Cherian, an internal-medicine physician in Baltimore, told Insider.

In general, older adults are less expected to experience side effects than younger people, since immune response usually weakens gradually with age, according to FDA’s analysis

Typically, the second dose amplifies the immune response to the virus which justifies the increasing intensity of side effects after the last shot is administered.

Vaccines stimulate the human body to produce antibodies, however, since immune systems can not distinguish between an actual infection and a vaccine-induced response, inflammatory chemicals are released automatically as part of a defence mechanism.

The insider report said some 57% of people younger than 65 developed side effects, compared with 48% of those older than 65.

However, after the second Moderna dose, nearly 82% of people in the younger group developed side effects, compared with nearly 72% of older adults.

Both vaccines administered in Qatar are more than 90% effective against the novel coronavirus.

Read also: Pfizer-BioNTech vaccine ‘100% effective’ in 12 to 15 year-olds

“However, it’s difficult to compare the companies’ trials side-by-side, since they happened at different stages in the pandemic and in different geographic regions,” Insider added. 

Common side effects

Once a person gets the shot, “blood flow increases and immune cells rush to the scene. This can result in pain, redness, or swelling at the injection site,” Insider noted. 

Some 92% of Moderna participants and 84% of Pfizer participants reported pain at the injection site. Headache and fatigue were also among the most common side effects across all trials, FDA’s report said.

Around 69% of those who took the Moderna vaccine reported headaches compared with 55% of people who received the Pfizer vaccine.

In both Pfizer and Moderna’s trial, around 63% reported fatigue. 

Side effects and age

The FDA analysis showed that younger adults are more likely to experience side effects after getting inoculated than seniors. 

Read also: Real-life study shows Pfizer, Moderna vaccines are 90% effective

However, the two age groups recorded few similar experiences.  

Though older adults tend to have fewer side effects in general, health experts say this does not imply that vaccines won’t work as well among the elderly.

“For the Covid-19 vaccine, we’ve actually not seen decreased effectiveness as we get older, so that’s actually a really good thing,” D. Cherian stated, assuring young adults should not worry about feeling side effects.

“Dealing with a few side effects of some diarrhoea or some muscle aches is a much, much better thing to get than some of those serious, potentially life-threatening side effects of the Covid-19 infection,” Cherian said.