With the COVID vaccine rollout now well underway, over 68,000,000 doses have reportedly been administered across the U.S. This is promising news not only for our eventual safety and return to normalcy, but also for the sake of data collection: with a much bigger pool of vaccinated individuals than the initial trial groups provided, we’re learning a lot about what to expect from the vaccine itself. In particular, there’s one common experience that doctors now say people should “be prepared” for: many vaccine recipients have reported that the second dose elicits more side effects than the first. Read on to learn more about what you can expect, and for more essential vaccine news, The Pfizer CEO Says This Is How Often You'll Need a COVID Vaccine.While the evidence from the U.S. is currently anecdotal, a study in the U.K. found that the second dose of the COVID vaccine does indeed yield higher rates of side effects. The researchers reviewed data from 40,000 subjects, over 12,000 of whom had received both vaccine doses, and discovered that the rate of side effects rose the second time around. After a single dose, 37 percent of recipients reported local side effects including pain or swelling near the site of the injection, and 12 percent reported at least one whole-body side effect in the days following their injection. Following the second dose, those numbers rose: 45 percent reported experiencing local side effects, and 22 percent reported broader effects.Doctors explain that this escalation of side effects makes sense, given how the vaccines work. "The second vaccine [dose]—think of it as having that hit to your immune system, and your immune system now recognizes the vaccine, so it does its job," Kavita Patel, MD, a medical contributor for NBC News recently told Al Roker. "Just be prepared," she said, adding that she had personally experienced side effects following her own second dose of the COVID vaccine.Bill Moss, MD, a pediatrician and professor of infectious disease epidemiology at the Johns Hopkins Bloomberg School of Public Health in Baltimore agreed with Patel’s assessment. "The second dose is really like a booster dose," he said. "The immune system is seeing the vaccine for the first time with the first dose and is reacting to that, and the cells of the immune system are recruited to kind of recognize that spike protein (the part of the coronavirus that the vaccine affects). So when the body's immune system sees [the vaccine] a second time, there are more cells and there's a more intense immune response, resulting in those side effects," explained Moss.However, if you don’t experience side effects, that’s perfectly normal, too. "If you don't have a reaction, you don't need to worry that it didn't work,” explained Patel. “Every human and body is different." Read on for everything you need to know about the potential side effects, and for promising news on another vaccine, This Other Vaccine Could Already Be Protecting You From COVID, Study Says. The side effects are usually mild. The most commonly reported side effects following COVID vaccination are pain or swelling at the injection site, chills, headache, fever, and fatigue, according to the CDC. The bright side? Most people who experience these symptoms say that their reactions have been mild to moderate, and are made better with the help of over-the-counter medications like ibuprofen or acetaminophen. And for the latest COVID news delivered straight to your inbox, sign up for our daily newsletter. They don’t last long. According to White House COVID advisor Anthony Fauci, MD, the vaccines’ side effects also tend to be short-lived. You can expect your discomfort to last up to 48 hours, with most side effects ceasing after 24 hours. And for more vaccination tips, Dr. Fauci Says You'll Easily Get a Vaccine Appointment After This Date. You can help track side effects using your smartphone. Just like the U.K., the U.S. has a robust vaccine monitoring system for tracking potential side effects. If you do experience discomfort, the CDC asks that you enter your symptoms into the v-safe app, a program designed to collect data on the rollout. Once you register, you can expect prompted health check-ins following your appointment and “depending on your answers, someone from CDC may call to check on you and get more information,” the health authority explains. There’s a lot of misinformation out there about the side effects. While it’s true that certain side effects are somewhat common after vaccination, there are several myths about side effects as well. Some have incorrectly suggested that the COVID vaccine can change a person’s DNA or infect you with coronavirus. These claims are patently false and scientifically impossible, according to the Mayo Clinic and other experts. And for more on vaccine side effects, Dr. Fauci Says These 2 Side Effects Mean Your COVID Vaccine Is Working.