The six British patients seemed to have little in common besides this: Each was dealing with kidney failure, and each had tested positive for the coronavirus.

They were among scores of virus-stricken people showing up at Addenbrooke’s Hospital in Cambridge in the early weeks of April. Had they lived in the United States instead of the United Kingdom, the link that allowed the contagion to spread among them might have slipped by unnoticed.

But the U.K. had done something in the early days of the pandemic that the United States and many other nations had not. It funded a national push to repeatedly decode the coronavirus genome as it made its way across the country. The process reveals tiny, otherwise invisible changes in the virus’s genetic code, leaving a fingerprint that gives scientists valuable glimpses into how the disease is spreading. It’s a cutting-edge technique that