There’s a chance that if you have already been exposed to SARS-CoV-2, it can worsen your response to a subsequent exposure. For example, the first time someone is exposed to the virus, they often have only a mild infection, but if they encounter it a second time, it can become deadly. Researchers believe this is due to the antibodies your immune system generates after the first exposure to the virus, which start to diminish over time. If they drop to a low enough level they’re not able to fight off the virus, but instead, during a second exposure, they can inadvertently assist the virus in infecting cells. This is what often happens with Dengue virus. It’s called antibody-dependent enhancement, and researchers are investigating whether past exposure to this coronavirus could worsen the symptoms of COVID-19 after a subsequent exposure to the virus.
This is a potential concern with a CoV-2 vaccine. An international consortium of European and US healthcare agencies is overseeing the dozens of anti-coronavirus vaccines in development around the world. This concern is one of the first things they want to address in animal models before a vax moves into human trials.
At this point, we even need to be careful about "herd immunity" until we know more about this novel pathogen.
In the meantime, remember to wash your hands and avoid French kissing strangers!
Making Sense of Medical Science (MSMS) 
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