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WATCH: Medical experts testify on COVID-19 vaccine

The U.S. House held a hearing on the nation’s vaccine efforts Wednesday with experts from the National Academy of Sciences.

Watch the hearing in the player above.

One of COVID-19’s scariest mysteries is why some people are mildly ill or have no symptoms and others rapidly die — and scientists are starting to unravel why.

An international team of researchers found that in some people with severe COVID-19, the body goes rogue and attacks one of its own key immune defenses instead of fighting the coronavirus. Most were men, helping to explain why the virus is hitting men harder than women.

And separate research suggests that children fare better than adults thanks to robust “first responder” immune cells that wane with age.

They’re the latest in a list of studies uncovering multiple features of the immune system’s intricate cascade that can tip the scales between a good or bad outcome. Next up: Figuring out if all these new clues might offer much-needed ways to intervene.

“We have the knowledge and capability of really boosting many aspects of the immune system. But we need to not use the sledge hammer,” cautioned Dr. Betsy Herold of New York’s Albert Einstein College of Medicine, who co-authored the child study.

Adding to the complexity, people’s wildly varying reactions also reflect other factors, such as how healthy they were to begin with and how much of the virus — the “dose” — they were exposed to.

“Infection and what happens after infection is a very dynamic thing,” said Alessandro Sette, a researcher at the La Jolla Institute for Immunology in San Diego, who is studying yet another piece of the immune response.

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