For a fraction of people, getting these first COVID-19 vaccines could be unpleasant—more than the usual unpleasantness of getting a shot. They might make you feel sick for a day or two, even though they contain no whole viruses to actually infect you. Both the Pfizer/BioNTech and Moderna vaccines are quite “reactogenic”—meaning they stimulate a strong immune response that can cause temporary but uncomfortable sore arms, fevers, chills, and headaches. In other words, getting them might suck a little, but it’s nowhere near as bad as COVID-19 itself.
Reactogenicity happens to some degree with all vaccines and is not in itself a safety concern. Vaccines, after all, work by tricking the body into thinking it has been infected, and these “symptoms” are an indication it has successfully done so. The fever, fatigue, and other signs we associate with colds or flu or even COVID-19 are typically caused by our immune responses, not the virus itself. “A reactogenic vaccine is not the same thing as an unsafe vaccine,” says Saad Omer, a vaccinologist and the director of the Yale Institute for Global Health.
Compared with existing vaccines, the two COVID-19 ones from Pfizer/BioNTech and Moderna are a little more reactogenic than flu vaccines but are roughly on par with the shingles vaccine, which can interfere with daily life for a couple of days in some people. (Moderna’s also seems to be a little more reactogenic than Pfizer’s, possibly because it’s a larger dose.) These vaccines have enough of a kick that the CDC suggested hospitals stagger vaccinations among staff, so an entire unit isn’t out on a given day. Nursing homes as well are concerned about vaccinating all staff and residents at once, which could reduce staff availability at the same time residents need more care.