If you get any of the COVID-19 vaccines now authorized for emergency use in the US, you may have side effects (and more so after the second dose of the Pfizer-BioNTech or Moderna shots as opposed to the first). The symptoms, which are generally short-lived and last a day or two, may include headache, fatigue, injection-site reactions, fever, and muscle aches and pain. However, they are in no way contagious.
Sabrina Assoumou, MD, MPH, an infectious diseases physician at the Boston Medical Center and an assistant professor at Boston University's School of Medicine, explained that the side effects are a result of your body's normal immune response, and there is no live part of the COVID-19 virus in the vaccines.
"When I got vaccinated, for about 24 hours or 36 hours I had pain on my arm, the injection site where I got vaccinated. I felt really tired. And after my second shot, not only did I feel tired and have the pain, but I also had a little bit of a fever," Dr. Assoumou told POPSUGAR. "The reason why I was getting those symptoms was because my body was busy making all of those antibodies." It's what the immune system does when it recognizes something foreign in the body (though you should know that having zero side effects from the vaccine doesn't mean you're not building an immune response).
Antonio Crespo, MD, an infectious disease expert at Orlando Health, further explained to POPSUGAR that both types of vaccines available in the US right now tell your body how to create harmless copies of the spike protein found on the surface of the SARS-COV-2 virus, which instigate the production of antibodies; the only difference is how that information is transported into your cells.
The mRNA vaccines from Pfizer and Moderna teach immune cells how to make the SARS-COV-2 virus spike protein through instructions delivered in "tiny lipid balls," Dr. Crespo explained. After the protein is made, the cells break down the instructions and dispose of them. The single-shot Johnson & Johnson vaccine uses a special type of virus, adenovirus type 26 (Ad26), as a vector to deliver a piece of the DNA that makes the spike protein. Ad26, according to the FDA, can cause pink eye or a cold, but it has been modified for the vaccine and cannot cause illness.
"The vaccine does not have a version of the virus that you can spread," Dr. Assoumou said. So, you don't have to worry about "getting" a virus from someone in your household who has symptoms after the vaccine or vice versa. However, it is important to note that it's still possible to get COVID-19 in between doses of the mRNA vaccines — and the CDC only classifies someone as fully vaccinated two weeks after the second mRNA vaccine or the Johnson & Johnson dose.
POPSUGAR aims to give you the most accurate and up-to-date information about the coronavirus, but details and recommendations about this pandemic may have changed since publication. For the latest information on COVID-19, please check out resources from the WHO, the CDC, and local public health departments.