Side-effects have been reported for all three vaccines approved for emergency use in the US but most are mild and short-livedAs more people become eligible to receive a coronavirus vaccine many are asking what side-effects they should expect, and if there are differences between the side-effects of the vaccines. The short answer to both questions is yes – the details are below – though any discomfort pales in comparison with contracting Covid-19. We used clinical trial data gathered by the US Food and Drug Administration (FDA) to explore the kinds of side-effects most commonly associated with the three vaccines currently authorized for emergency use in the US. Those vaccines were developed by Pfizer and BioNTech, Moderna and the National Institute of Allergy and Infectious Disease (Niaid) and most recently by Johnson & Johnson. What are the common side-effects? For all the vaccines, the most common side-effects include: Soreness where the vaccine is injected. Fatigue. Headache and muscle soreness. Less common side-effects can also include nausea, chills and fever. The vast majority of symptoms cause discomfort, but not a total disruption of your daily habits. Are Covid-19 vaccines safe? Yes. Their safety is tested in large trials of tens of thousands of people, then the FDA and the Centers for Disease Control and Prevention (CDC) continue to monitor vaccine safety data, including side-effects, after the vaccines are authorized. These are sometimes referred to as phase IV trials. That monitoring goes hand-in-hand with reporting through several vaccine safety registries. These ongoing studies can help identify the rarest of side-effects, and pinpoint people who may have special sensitivities to the vaccine, such as a potential for an allergic reaction. One of the key numbers included in the graphs below is the rate of people who experienced side-effects after receiving a “placebo”, or an injection of saline instead of the vaccine. People involved in the trials did not know whether or not they received the vaccine. This helps researchers understand the background rate of these side-effects in the population. Moderna vaccine side-effects Formally called mRNA-1273, this vaccine was developed by Moderna in partnership with the Niaid, but most people simply know it as the Moderna vaccine, which is a two-dose regimen spaced 28 days apart. A clinical trial involving more than 30,000 participants across 99 sites in the US found the vaccine was safe and effective, and protected people against Covid 94.1% of the time. Among those trial participants, 15,168 people received the vaccine and the rest received a placebo. Moderna dose 1 side-effects We used results from the vaccine’s trials to describe how likely it is for people aged 18 to 64 to experience a given side-effect within one week of a dose of the vaccine. On average, these symptoms cleared up within three days, and often less. default Moderna dose 2 side-effects A key feature of the two-dose regimen is that people are more likely to experience side-effects after the second dose. default Pfizer vaccine side-effects A vaccine developed by Pfizer with the pharmaceutical name BNT162B2 uses mRNA technology, much like Moderna’s vaccine. Their trial used 152 sites around the world. While the majority (130) were in the US, trial sites were also located in Brazil, Argentina and South Africa. More than 43,000 people were involved in the trial. The vaccine was found to be 95% effective at preventing Covid-19. Pfizer’s vaccine uses a two-dose regiment spaced 21 days apart. Pfizer dose one side-effects Unlike Moderna, Pfizer studied side-effects in two separate age groups: people between aged 16 and 55, and people older than 55. Because people older than 55 are slightly less likely to experience side-effects, the younger group is presented here. default Pfizer dose two side-effects Like the Moderna vaccine, some side-effects were more common after the second dose. default Johnson & Johnson side-effects The most recent vaccine authorized in the US is from the Johnson & Johnson subsidiary Janssen. Johnson & Johnson’s trial included more than 40,000 people across 19 geographic regions. Importantly, this included South Africa, where the vaccine was found to be slightly less effective against the B1351 variant. The FDA found this vaccine is more than 66% effective at preventing moderate to severe Covid-19. While this efficacy rate is lower than the two previously discussed vaccines, it still gives near perfect protection against hospitalization and death, and provides advantages in fighting the pandemic. On 13 April, the FDA and CDC recommended that states pause the administration of the Johnson & Johnson vaccine after reports that six women developed rare and severe blood clots. While serious, these side effects are exceedingly rare. More than 6.8m doses have been administered nationally. Johnson & Johnson side-effects The Johnson & Johnson vaccine requires only one dose, has a generally lower rate of side-effects. default What causes the side-effects? Side-effects are a sign the vaccine is prompting your body to mount an immune response. They may be uncomfortable, but can also be a sign the vaccine is working as intended. Side-effects are caused by the release of chemicals in the body which signal to the immune system it is time to mount a response. These naturally occurring chemicals are called cytokines and chemokines. Though there is not a one-to-one correlation between side-effects and an immune response, side-effects are an expected part of the process. “What we look for as vaccinologists is the Goldilocks response,” said Dr Greg Poland, editor-in-chief of the medical journal Vaccine and head of the Vaccine Research Group at the Mayo Clinic. Poland also consults for major vaccine manufacturers. “We don’t want too little” immune response, “we don’t want too much, we want just enough,” said Poland. The balance Poland is describing is sometimes referred to as between “immunogenicity” and “reactogenicity”, or the propensity to prompt an immune response versus the tolerability of side-effects. Why should I trust this data? In vaccine science, these often mild side-effects are described as “adverse events”. In Covid-19 vaccine studies, researchers collected information on everything from hip fractures to heart attacks to monitor the safety of these vaccines. “We have a very, very robust system in the US,” to track adverse vaccine events, said Poland. “It’s how we were able to pinpoint a risk of anaphylaxis in specific kinds of people occurring at the rate of 2.4-4.5 per million, something you could never have done in this kind of timely manner in the past.” Importantly, the vast majority these side-effects are not related to vaccines, but are nevertheless documented to ensure there is not a pattern, and to verify the safety of one of the only medical interventions given to healthy people. The FDA then produces its own analysis of vaccine trials, and the data is presented to an independent panel of experts, who review it and make a recommendation on whether to authorize the vaccines based on the results. The FDA analyses presented to the committee are public, and can be viewed here, as can meetings of this expert panel.