But since the cells make only a piece of the virus, and not the whole pathogen itself, we don’t get sick. When human cells receive these instructions, they churn out copies of spike protein. In the case of the authorized COVID-19 vaccines-Pfizer, Moderna, and Johnson & Johnson-all contain a genetic blueprint for manufacturing spike proteins, which sit on the surface of the coronavirus and enable it to infect human cells. Still, it’s important to put these side effects in perspective, says Jacobson, “because these are mild, temporary, and transient reactions that disappear within a few days.” What causes the immune reactions? They may experience itching or hives, or a range of flu-like symptoms, such as chills and fever, headaches, or debilitating fatigue, that can leave them bedridden for a day or two. Some feel a soreness at the injection site on their arm. Studies do suggest that up to seven out of ten people getting their second shot have some type of reaction. Case in point, in studies of the Pfizer/BioNTech vaccine, 23 percent of people aged 16 to 55 who received the placebo complained of fatigue after their second jab, and 24 percent noted headaches. It’s a lesson that may be useful to medical professionals, who can reassure patients that most side effects are normal and predictable-and may not even be caused by the shot. “We’ve seen this in the military, when young recruits, who think they can tolerate anything, faint when they get the injections, because their body overreacts,” says Jacobson. Recent studies show some side effects, even ones from the COVID-19 vaccines, aren’t due to the shots at all, but to our own fears. The take-home message here, says Jacobson: “It’s easy to confuse an allergic reaction with nervousness or emotions or even stomach upsets from anxiety.” Half of these people had received the placebo and yet they complained of the worst headaches, or worst fever, of their lives. “About five percent said they got sicker than they ever had been in their entire life,” says Jacobson. When the researchers unblinded the study to see who received the vaccine versus the placebo, the side effects were split equally between the two groups, says Robert Jacobson, medical director for the population health science program at the Mayo Clinic. The study involved more than 300 veterans over the age of 65 who were given either a flu shot followed two weeks later by a placebo injection of salt water, or a placebo shot followed two weeks later by the real vaccine. To address this issue, in 1991, a group of scientists in Minnesota-at the Department of Veterans Affairs and the Mayo Clinic-devised an experiment to see just how frequent these unpleasant reactions were. Side effects can be a powerful deterrent stopping people from getting vaccinated.
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