Ever since Covid-19 first began to spread, the question of how many people are infected with the disease has remained difficult to answer. Official case counts, it is widely acknowledged, represent only the tip of the iceberg. Meanwhile, epidemiological data collected by the open-source project Nexstrain.org, along with a recent preprint in Cell that is currently awaiting peer review, suggest that the strains of SARS-CoV-2 we’re seeing now may be more transmissible than before.

Just how widespread is infection actually? In Spain, a broad survey of the country’s population found that about 5 percent—that is, 2.3 million people—were infected. Prevalence varied according to occupation (health workers, 10 percent) and age (children, 3 to 4 percent). The actual rate of infection, the study suggests, was about ten times the roughly 250,000 cases picked up by standard genome tests.

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Originally published in Forbes (July 22, 2020)