Marshaling these reinforcements is important, but the influx of cells and molecules makes the upper arm swollen and sore. Once they spot the nanoparticles, these cells dispatch molecular alarms called cytokines that recruit other immune cells to the site of injection. These strange, fatty spheres don’t resemble anything naturally present in the body, and they trip the sensors of a cavalry of fast-acting immune cells, called innate immune cells, that patrol the body for foreign matter. To ensure safe passage of mRNA into cells, the vaccine makers swathed the molecules in greasy bubbles called lipid nanoparticles. Read: What’s the use of a pretty good vaccine? Once delivered into the upper arm, the mRNA instructs the body’s own cells to produce a coronavirus protein called spike-a molecule that elicits powerful, infection-fighting antibody responses in people battling COVID-19. The Pfizer and Moderna vaccines accomplish that pedagogy via a genetic molecule called mRNA that’s naturally found in human cells. Vaccines impart these same lessons without involving the disease-causing pathogen itself-the immunological equivalent of training wheels or water wings. When the immune system detects a virus, it will dispatch cells and molecules to memorize its features so it can be fought off more swiftly in the future.

Fortunately, side effects resolve quickly, whereas COVID-19 can bring on debilitating, months-long symptoms and has killed more than 2 million people. “By the second vaccine, it’s already amped up and ready to go,” Jasmine Marcelin, an infectious-disease physician at the University of Nebraska Medical Center, told me.

The body’s encore act, uncomfortable though it might be, is evidence that the immune system is solidifying its defenses against the virus. When hit with the second injection, the immune system recognizes the onslaught, and starts to take it even more seriously. My husband, who’s a neurologist at Yale New Haven Hospital, is one of many who had a worse experience with his second shot than his first.īut much like any other learning process, in this one repetition is key. 2 is more likely to pack a punch-in large part because the effects of the second shot build iteratively on the first. But the two COVID-19 vaccines cleared for emergency use in the United States, made by Pfizer/BioNTech and Moderna, already have reputations for raising the hackles of the immune system: In both companies’ clinical trials, at least a third of the volunteers ended up with symptoms such as headaches and fatigue fevers like my husband’s were less common.ĭose No. Side effects are a natural part of the vaccination process, as my colleague Sarah Zhang has written. All this misery was a sign that the immune cells in his body had been riled up by the second shot of a COVID-19 vaccine, and were well on their way to guarding him from future disease. And as I lay next to him, cinching blanket after blanket around his arms, I felt an immense sense of relief.

For hours, he had been tossing in bed, exhausted but unable to sleep, nursing chills, a fever, and an agonizingly sore left arm. on Thursday morning, I woke to find my husband shivering beside me.

Moderna side effect sweating