As the vaccine rollout continues across America, people are rolling up their sleeves—and bracing for side effects, especially after the second dose of the two-part vaccines developed by Moderna and Pfizer/BioNTech. As Katherine J. Wu reported for the Atlantic in February, the second dose of a vaccine can prompt surprisingly strong side effects like fever, chills and fatigue because the immune system is ready to use the skills it learned after the first dose.
Now the Centers for Disease Control and Prevention has released data about how often people experienced side effects after the Pfizer/BioNTech and Moderna vaccines. The data come from electronic diaries that clinical trial volunteers kept for seven days after each injection in order to record side effects. The CDC's dataset is broken down by age and shows that older people tend to report fewer side effects than younger people.
Pain at the injection site is the most common side effect, according to the CDC report. After the first dose of either a Moderna or Pfizer/BioNTech vaccine, just over 70 percent of older people reported pain in their arms, and just over 80 percent of younger people reported pain. Pain was more common after the second dose for recipients of the Moderna vaccine, but slightly less common after the second dose of the Pfizer/BioNTech vaccine. Less than five percent of people who received the Moderna vaccine reported “Grade 3” pain, which is defined either by pain that prevents a person from completing normal daily activities, or indicates a pain reliever is needed to treat symptoms.
Other side effects at the injection site, like redness and swelling, each affected less than ten percent of people who got the Pfizer/BioNTech vaccine, and less than 20 percent of people who got the Moderna vaccine.
One of the most-talked-about side effects from the vaccines is a fever. As Patricia Mandatori, who lives in Los Angeles, tells Kaiser Health News’ Arthur Allen, the side effects "felt like a truck hit me. When I started to feel rotten I [said], 'Yay, I got the vaccination.' I was happy. I felt relieved."
The data show less than 20 percent of younger people who participated in the study reported fevers of higher than 100.4 degrees Fahrenheit after their second dose of either vaccine.
For Pfizer/BioNTech’s vaccine, only 3.7 percent of participants had fevers after the first dose, and 15.8 percent reported fevers after the second dose. In people over 55 years old, 1.4 percent reported fevers after the first dose of Pfizer/BioNTech’s vaccine, and 10.9 percent reported fevers after the second dose.
Less than one percent of people who received the Moderna vaccine reported fevers after the first dose, regardless of age. After the second dose, 17.4 percent of younger participants and 10.2 percent of participants 65 and older reported fevers.
Chills were another common side effect. About half of younger Moderna recipients, and about a third of younger Pfizer recipients, reported chills after their second dose.
Dozens of vaccine trial participants also reported lymphadenopathy—swollen lymph nodes—after receiving their jabs. The effects tended to appear around the arm and neck within two to four days of the vaccination, and lasted for an average of ten days after the Pfizer/BioNTech vaccine, and one to two days after the Moderna vaccine.
The data also show that not everyone will have severe side effects. Having a strong reaction “is an interesting but, in a sense, not vital question," says Vanderbilt University Medical Center infectious disease specialist William Schaffner to Kaiser Health News. For the most part, he says, "Don't worry about it."
The severity of side effects doesn’t necessarily relate to how strongly the vaccine will protect a person from Covid-19. A significant number of people in the vaccine trials didn’t report side effects at all, and the two-dose vaccines showed 95 percent efficacy. University of Pennsylvania pediatrician Paul Offit, who specializes in infectious disease and is a member of the FDA’s vaccine advisory panel, tells the New York Times’ Tara Parker-Pope, “That proves you don’t have to have side effects in order to be protected.”