What to expect when you’re injected: Vaccine side effects explained

Vaccinations for Covid-19 are now underway in the U.S. Two vaccines – one from Pfizer and BioNTech, one from Moderna — have received emergency use authorizations by the Food and Drug Administration. Both vaccines have been deemed safe, but they can cause some short-term side effects.

Vaccines are designed to invoke an immune response that builds protection without causing a serious infection. Traditionally, vaccines are made with viral material which prompts the immune system to mount a defense. This process releases chemicals that cause physical symptoms — such as pyrogens, which are inflammatory chemicals that can cause fever. 

It’s not just vaccines that can cause such a reaction, said Lee Riley, chair of the Division of Infectious Disease and Vaccinology at the University of California, Berkeley. “If you fall down and you get dirt into your skin, some people will develop fever afterwards, if enough immune cells get stimulated.”

The Pfizer and Moderna Covid-19 vaccines are not made with viral material. They are the first vaccines to use messenger RNA technology, known as mRNA, to confer immunity. This essentially adds an extra step. The mRNA has the genetic instructions to create the antigen — which for Covid-19 is the virus’ telltale spike protein. When injected, the body will begin creating this spike protein.

Once the antigen is present in the body, the immune system handles it the same way it would any other invader. That is, the mRNA vaccines don’t cause side effects in a new or different way. 

“The biggest concern is that people might think that the vaccine could actually give them Covid-19,” said Matthew Laurens, professor at the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine. “It absolutely can’t, especially with these mRNA vaccine products that only contain the genetic code.” He stressed that educating the public is crucial, so when people experience common side effects they will know that it just means the vaccine is working.