COVID-19 is caused by the coronavirus SARS-CoV-2. It spreads through the air and infects the respiratory tract. While some people only feel mild cold-like symptoms, others develop pneumonia, blood clots, or long-lasting complications. Vaccines became available in 2020 and rapidly reduced the number of severe cases. A study estimated that in two years vaccination in the United States prevented more than 2.5 million deaths. Booster shots keep protection strong as the virus changes.

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