The views and opinions expressed here are those of the authors and do not necessarily reflect the position of either Johns Hopkins University and Medicine or the University of Washington.
By the end of December 2020, approximately 3 million Americans had received their first dose of one of two vaccines to prevent Covid-19 following Emergency Use Authorization by the U.S. Food and Drug Administration. This is a remarkable achievement, although much more needs to be done to get us to the other side of this pandemic. These vaccines – one developed and produced by BioNTech and Pfizer and the other by the National Institutes of Health and Moderna – are based on delivering messenger RNA (mRNA), which directs our muscle cells to produce the spike surface protein of SARS-CoV-2 and induce protective immune responses. While most Americans say they will get a Covid-19 vaccine, many say they are hesitant in part because of potential side effects, both known and unknown. Rumors and conspiracy theories about Covid-19 vaccines are circulating; here, we separate fact from fiction.
What do we know so far about side effects of the mRNA vaccines? The best information comes from the large phase 3 trials that demonstrated the safety and efficacy of these vaccines and served as the basis for the Emergency Use Authorizations. The most common side effects of both vaccines are what is called reactogenicity. These are expected side effects and caused by local inflammation (redness and swelling) at the site of injection or more generalized reactions such as fever and muscle aches. Inflammatory reactions are part of the body’s immune response to the vaccine, a sign that the vaccine is doing what is it designed to do.
For both the Pfizer and Moderna vaccines, these reactogenic side effects were mild to moderate, occurred up to two days after vaccination, and do not have any long-term consequences. For the Pfizer mRNA vaccine, the most common side effects were soreness or redness at the site of injection (84% of vaccine recipients), fatigue (63%), headache (55%), muscle pain (38%), chills (32%), joint pain (24%), and fever (14%). Severe adverse reactions – which include any severe disease after vaccination whether related to the vaccine or not – occurred in less than 5% of vaccine recipients and were more common after the second dose and less common in adults older than age 55. Most severe adverse reactions were not thought to be associated with the vaccine, with the possible exception of swollen lymph nodes, although this was quite rare (0.3% of vaccine recipients). Four vaccine recipients developed Bell’s palsy, a paralysis of the facial nerve, but this was thought to be consistent with the expected rate of Bell’s palsy in the general population. Side effects following receipt of the Moderna mRNA vaccine were similar to those reported for the Pfizer vaccine but with slightly different frequencies. The most common side effects were soreness or redness at the site of injection (92%), fatigue (69%), headache (63%), muscle pain (60%), joint pain (45%), and chills (43%). Severe adverse reactions occurred in less than 10% of vaccine recipients and also were more common after the second dose and less common in adults older than age 65. Most severe adverse reactions were not thought to be associated with the vaccine, again with the possible exception of swollen lymph nodes, although this too was quite rare (1.1% of vaccine recipients). Slightly more vaccine recipients (1.5%) developed an allergic reaction compared to placebo recipients (1.1%), although none were severe. Three vaccine recipients and one placebo recipient developed Bell’s palsy.
What about those allergic reactions? During the phase 3 trials, in which 18,858 people received the Pfizer mRNA vaccine and 15,180 people received the Moderna vaccine, no one developed a severe allergic reaction, what we call an anaphylactic reaction. Anaphylactic reactions most commonly occur in some people after exposure to certain foods such as shellfish, insect stings, medications, or latex. We first learned of anaphylactic reactions to the Pfizer mRNA vaccine after two healthcare workers in the United Kingdom developed such a reaction shortly after vaccination. Both had a history of having severe allergic reactions. Since then, several people in the United States have also had anaphylactic reactions to the mRNA vaccines. Fortunately, these reactions can be treated with epinephrine or corticosteroids. For this reason, the mRNA vaccines should only be administered in a setting capable of managing and treating anaphylactic reactions. We still do not know the exact cause of these reactions but some think it may be a reaction to the fatty coat that surrounds and protects the mRNA in the vaccine, specifically polyethylene glycol, a compound found in many products including skin creams and toothpaste and that has been associated with allergic reactions. People who have an immediate allergic reaction to the first vaccine dose should not receive additional doses of either mRNA Covid-19 vaccine.
We will learn much more about side effects associated with the mRNA vaccines as more people are vaccinated. The U.S. Centers for Disease Control and Prevention has set up a smartphone-based tool, called V-safe, that uses text messaging and web surveys to provide personalized health check-ins after vaccination and through which vaccine recipients can tell the CDC of any side effects they have experienced. There are still things to be learned. We still need to know more about adverse reactions that are rare or require longer follow-up to be detected, which will come with time and as more people are vaccinated. Although there is nothing to suggest this, we do not yet know if there is a risk of vaccine-enhanced disease, i.e., more severe Covid-19 in people who were previously vaccinated, particularly as immunity wanes over time. And importantly, we do not yet have sufficient information on vaccine safety in certain groups of people, including children younger than 16 years of age, pregnant and lactating women, and immunocompromised individuals. There is no reason to think Covid-19 vaccines will be less safe in these groups but more information is needed to be certain. We can be certain, however, that these vaccines do not alter our DNA, cause sterility, or infect one with SARS-CoV-2. These conspiracy theories are falsehoods that should not be believed.
We are fortunate to have at least two highly protective and safe vaccines against Covid-19. All vaccines have side effects, and the two mRNA vaccines for Covid-19 are no different. These side effects are similar to what people may experience after being vaccinated for flu or shingles. They may cause discomfort for a day or two but they can be managed easily and do not have long-term consequences. We cannot say the same for Covid-19.