Side effects of the coronavirus vaccines developed by Pfizer and Moderna are common, and they become more frequent after the second dose. Although they’re mild in most cases, they also can be more severe than those for flu shots.
Here’s what trial participants experienced:
Fatigue: Roughly 63% of the Pfizer trial’s 38,000 participants reported fatigue as a side effect, according to the Food and Drug Administration’s vaccine advisory panel. Meanwhile, over 68% of Moderna’s 30,350 trial participants reported fatigue.
Injection site pain: 84.1% of Pfizer’s trial participants experienced injection site pain, redness, and swelling, as did 91.6% of Moderna’s trial participants.
Headaches: Just over 55% of Pfizer’s volunteers reported headaches, while 63% of Moderna’s respondents reported headaches. Participants in both trials reported headaches more frequently after the second dose.
Muscle Pain: 38.3% of Pfizer’s volunteers reported muscle pain, compared to 59.6% of Moderna’s trial participants.
Chills: 31.9% of Pfizer’s trial participants reported chills, as did 43.4% of Moderna’s trial participants.
Joint pain: 23.6% of Pfizer’s volunteers reported joint pain, while 44.8% of Moderna’s volunteers reported the same.
Fevers: Only slightly more than 14% of Pfizer’s trial participants experienced fevers, while Moderna’s trial results didn’t report any instances of volunteers experiencing fevers.
It’s important to note that volunteers in both trials who received placebo shots, instead of the vaccine, also reported some mild side effects, albeit less frequently. In their analysis of the Moderna vaccine, the FDA’s vaccine experts reported that fatigue was the most common side effect in placebo recipients as well as vaccine recipients. About 36% of Moderna volunteers who got the placebo reported fatigue, 36.5% reported headaches, and 9.5% reported chills.
In the Moderna report, FDA vaccine experts also said the frequency of nonfatal but severe negative effects were equally low, 1%, in vaccine and placebo recipients. Volunteers who received the placebo shot were at a slightly higher risk of pneumonia and pulmonary embolism, or a blood clot in the lung.
The frequency of severe but nonfatal side effects reported in the Pfizer trial was also very low in vaccine and placebo groups, indicating that the shots did not put volunteers at a considerably higher risk of serious illness. Participants who received the placebo shots were at slightly higher risk than those who received the vaccine of developing pneumonia, atrial fibrillation, and syncope, or fainting.
The Pfizer and Moderna vaccines appeared to have a significant yet nonfatal side effect in common — Bell’s Palsy, a type of temporary paralysis in the face that usually resolves itself within about six months. In Moderna’s trial, three cases of Bell’s Palsy were reported in participants who received the vaccine, compared to one who received the placebo. Meanwhile, four cases of Bell’s Palsy were recorded in the Pfizer vaccine recipients, with none reported in the placebo group.
While serious side effects appear rare, regulators will have to monitor the vaccine’s performance continually over the coming months.
Some U.S. healthcare workers who have received the first doses of the Pfizer vaccine this week have experienced allergic reactions to the shots shortly after the first dose was administered. An Alaskan healthcare worker suffered what is being called an “allergic reaction” to the new COVID-19 vaccine within 10 minutes of being given the dose on Tuesday. The worker, who has only been identified as a middle-aged woman, is doing “well,” and the problem resolved shortly after the onset with the help of an epinephrine injector, or EpiPen.
Earlier this month, after the United Kingdom first began the rollout of the Pfizer vaccine, two National Health Service workers suffered allergic reactions that required them to use epinephrine injectors. The NHS reported they are doing fine, and England’s Medical and Health products Regulatory Agency launched an investigation into the causes of the reactions with Pfizer’s support.
Pfizer’s coronavirus vaccine works similarly to Moderna’s by using a technology called mRNA, a piece of genetic code that instructs cells to create a protein that then triggers an immune response.
The Pfizer vaccine has been deployed to hospitals and long-term care facilities across the U.S. Health and Human Services Secretary Alex Azar told reporters Wednesday that “shipments of [the Pfizer] vaccine will have been delivered to every delivery site identified by public health jurisdictions for our first wave of shipments” by the end of the day.
Moderna’s vaccine will follow closely behind. The FDA’s vaccine advisory panel will meet Thursday to determine whether the vaccine should be granted emergency use authorization, the same permit granted to Pfizer last week. HHS has allocated nearly 5.9 million doses to be shipped out within 24 hours of authorization.
Because the Moderna vaccine can be stored in less-extreme cold than the Pfizer vaccine, the Moderna vaccine may be easier to distribute to areas without access to specialized freezers.
