More than 600 million cases worldwide: COVID-19 has changed the world.Though there are still those who've dodged it.COVID-19 "superdodgers" who've been exposed to the virus plenty but never got sick.The Centers for Disease Control and Prevention estimates nearly half the population has never caught the virus and some scientists are working to learn why.At the University of California San Fransisco and Rockefeller University in New York, researchers are studying to see if genetics play a role.At Marquette University, assistant professor Dr. Nilanjan Lodh studies human genetics and infectious disease."There might be something physiologically going on there, which is good to know," Lodh said. He thinks physicians could use the genetic information for future medicine, he doesn't believe people are simply born with superdodger immune systems."Kind of like the course of your actions throughout your life, that helps the immunity build up slowly," Lodh said. "It's not like one-day stuff. You're acquiring that immunity over time."Lodh said what we eat, how often we exercise, where we live and what viruses we've been exposed to all contribute to one's current immunity.Dr. Mary Beth Graham works in the division of infectious disease at Froedtert and the Medical College of Wisconsin."I find this to be interesting science. I don't know that it turns into that we can make it translational," Graham said. "They're looking at people's HLA type to see, is there something about that HLA type which makes them more susceptible?"HLA are proteins found on most human cells, used by the immune system to recognize which cells belong in the body and which do not.Graham said she too has never gotten COVID-19, officially. Though she isn't sure if you can chalk it up to her genetics."You can't say with 100% assurance I've never seen the virus before because the virus has been here for a couple of years," Graham said. "So what if I could have been exposed a few years ago? Would those antibodies still be prevalent?"Graham also said testing genes is expensive and she doesn't see, when it comes to COVID-19, how it would help anyone."It is fascinating, but we can't go in and change our HLA type," Graham said. "It doesn't naturally lead into a new type of treatment."Both researchers said picking up the virus, may be inevitable."Sometimes you might get COVID but you don't realize you have it," Lodh said. So are there COVID-19 superdodgers out there? Nobody really knows for certain. Doctors still say your best chance at avoiding serious or even fatal symptoms is to get vaccinated, but Dr. Graham and Dr. Lodh both said it's probably unlikely anyone will dodge COVID-19 forever. "It's catchy," Graham said about the term 'superdodgers.'"I probably don't believe people are really dodging stuff. This virus is mutating very rapidly, compared to any other virus that we know," Lodh said. There is some additional certainty from our scientists.As flu, RSV and other illnesses spread this winter, experts warn having those viruses will lower people's immunity.Leaving everyone more vulnerable to COVID-19Watch the video above for the full story.

Video above: What are the most common side effects after getting the shot?Why do some people get side effects after COVID-19 vaccines?Temporary side effects including headache, fatigue and fever are signs the immune system is revving up — a normal response to vaccines. And they're common."The day after getting these vaccines, I wouldn't plan anything that was strenuous physical activity," said Dr. Peter Marks, the U.S. Food and Drug Administration's vaccine chief, who experienced fatigue after his first dose.Here's what's happening: The immune system has two main arms, and the first kicks in as soon as the body detects a foreign intruder. White blood cells swarm to the site, prompting inflammation that's responsible for chills, soreness, fatigue and other side effects.This rapid-response step of your immune system tends to wane with age, one reason younger people report side effects more often than older adults. Also, some vaccines simply elicit more reactions than others. That said, everyone reacts differently. If you didn't feel anything a day or two after either dose, that doesn't mean the vaccine isn't working. Behind the scenes, the shots also set in motion the second part of your immune system, which will provide the real protection from the virus by producing antibodies. Another nuisance side effect: As the immune system activates, it also sometimes causes temporary swelling in lymph nodes, such as those under the arm. Women are encouraged to schedule routine mammograms ahead of COVID-19 vaccination to avoid a swollen node being mistaken for cancer. Not all side effects are routine. But after hundreds of millions of vaccine doses administered around the world — and intense safety monitoring — few serious risks have been identified. A tiny percentage of people who got vaccines made by AstraZeneca and Johnson & Johnson reported an unusual type of blood clot. Some countries reserved those shots for older adults but regulatory authorities say the benefits of offering them still outweigh the risks. People also occasionally have serious allergic reactions. That's why you're asked to stick around for about 15 minutes after getting any type of COVID-19 vaccine — to ensure any reaction can be promptly treated.Finally, authorities are trying to determine whether temporary heart inflammation that can occur with many types of infections also might be a rare side effect after the mRNA vaccines, the kind made by Pfizer and Moderna. U.S. health officials can't yet tell if there's a link but say they're monitoring a small number of reports, mostly male teens or young adults.