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It is among the lingering mysteries of COVID-19: Why do some people catchit while others – sometimes even sleeping in the same bed – escape without symptoms? Some of the factors that explain these differences have long been clear: Older adults are more likely to fall seriously ill, particularly if they smoke, are obese or have diabetes. People never exposed to the coronavirus won't get sick, of course, and exposure to a high concentration poses a higher risk for infection. The virus itself matters, too. The delta variant seems to be more contagious than earlier ones. (Weintraub, 7/8)

On March 16, 2020, five days after the World Health Organization declared Covid-19 a global pandemic, Andrea Ganna, a geneticist at the Institute for Molecular Medicine Finland, took to Twitter to make an announcement: “We are launching the ‘COVID-19 host genetics initiative,’” he wrote. He asked other scientists to join him and institute director Mark Daly in probing the world’s stores of human DNA to help answer a complicated but pressing question: Why do some unlucky people infected by the coronavirus end up gasping for air in an intensive care unit, while many others catch and spread the disease without having so much as a cough? (Molteni, 7/8)

Children are at extremely slim risk of dying from Covid-19, according to some of the most comprehensive studies to date, which indicate the threat might be even lower than previously thought. Some 99.995% of the 469,982 children in England who were infected during the year examined by researchers survived, one study found. (Roland, 7/8)

For 234 COVID-19 patients, resting heart rates (RHRs) did not return to baseline until a mean of 79 days, reports a JAMA Network Open research letter yesterday. Step counts and sleep duration both took about a month to return to normal. The researchers derived their COVID cohort from the Digital Engagement and Tracking for Early Control and Treatment (DETECT) study and analyzed data from Mar 25, 2020, to Jan 24, 2021. As a comparison, they also looked at 641 participants who had acute respiratory illness but not COVID-19. (7/8)