Why do some people never get Covid-19?

As an intensive care unit doctor, I often find myself thinking about the apparent randomness of infectious disease. Two people go out to dinner and have the same meal; one ends up in the emergency room with food poisoning, but the other does not. The seasonal flu runs through an entire family, except for one individual who remains healthy.

- Daniela J. Lamas

Doctors look for the vulnerabilities that we can see to explain these outcomes, like age, vaccination status and underlying conditions, but we are often left without answers. The unpredictability of the coronavirus has made clear just how much we don’t know.

Standing at the bedside in the Covid-19 intensive care unit during the first wave, I wondered why young men without identifiable risk factors had become critically ill while their spouses and children were able to manage their symptoms at home.

Now physicians and researchers throughout the globe are asking, and attempting to answer, similar questions.

Dr Mayana Zatz was taking her usual stroll near her home in Sao Paulo, Brazil, when she realised she hadn’t seen one of her neighbours for several weeks. When she ran into his wife, Dr Zatz learnt that he had been sick at home with a high fever, a cough and flu-like symptoms. Even then, in February 2020, these were telltale signs of Covid-19. The woman was caring for him by herself, without a mask, and though she had expected to fall ill, too, she was feeling just fine. In the weeks that followed, Dr Zatz, a geneticist at the University of Sao Paulo, could not stop thinking about her neighbours. Why had the woman not become sick?

Dr Zatz spread the word on television that she wanted to study more discordant couples like her neighbours. To her great surprise, she was inundated with thousands of e-mails. Her neighbour’s story wasn’t so unusual, after all.

A global network of scientists, Dr Zatz prominent among them, believes crucial clues may lie in our genes.